so my question is whether adding these simple items in parallel can somehow affect the low frequencies too?

Unlikely.

I guess the capacitor and tweeter offer another current path for high frequency signal, but low freq. should remain unchanged, or is it?.

This is simply called "crossover". The simplest construction is to connect a tweeter with its series capacitor to woofer in parallel:

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: Ideally, the woofer should have a series inductor for HF filtering but in some applications it's not put. So I didn't illustrate it.

The series capacitor to the tweeter works as a blocker for LF signals, and the tweeter and the capacitor form a simple high-pass filter. Hence the name crossover. If the impedance of the tweeter is 8 Ohms and the series capacitor is 3.3 μF then the crossover frequency will be around 6 kHz. This means that almost anything above 6 kHz will go to the tweeter.

Could it be that adding the tweeter changes the impedance (or some other parameter) that the head unit sees and it changes equalization in response?

Unlikely. For low frequencies (i.e. below 500 Hz) the tweeter arrangement will have a relatively high impedance and therefore will not affect the net impedance of the load network (e.g. 4 Ohms in parallel with 500 Ohms is still around 4 Ohms).

To illustrate what I hear: I usually had this radio tuned with the treble adjustment to the max (And it was never enough, so I added the tweeters) and the bass adjusted about 3 notches bellow max, somewhat 5 notches over it's neutral point. After adding the tweeters I use treble one or two notches up and bass about three notches up and still think I hearing louder bass than before.

Normally, in sound engineering, above 3 kHz is considered as hi-mid and, in some cases, treble. So a crossover frequency of 6 kHz is somewhat insufficient for your application (Note that if the speaker impedance is lower then the crossover will go further. e.g. 12 kHz for 4 Ohms. 8 Ohms of impedance is just an assumption here).

Woofers have larger cone diameter and thus they push the air easily. So the bass frequencies can be heard easily by their nature. And also, for almost all genres, almost all music recordings have higher low frequency (below 500 Hz) energy compared to mids (500 Hz - 2-3 kHz) and trebs (2-3 kHz and above).

In conclusion,

• a crossover frequency of 6 kHz for a tweeter is a bit high. Adding capacitors in parallel will reduce the crossover frequency but we don't have any info about the tweeter speaker's response on its own. The speaker itself might not have a good response at lower frequencies (e.g. 3 kHz).
• The series inductor in woofer module might be missing. Remember that the series inductor should be placed to block high frequencies so that the woofer does not try to produce HF content (those speaker are generally terrible at those frequencies but who knows).

so my question is whether adding these simple items in parallel can somehow affect the low frequencies too?

Unlikely.

I guess the capacitor and tweeter offer another current path for high frequency signal, but low freq. should remain unchanged, or is it?.

This is simply called "crossover". The simplest construction is to connect a tweeter with its series capacitor to woofer in parallel:

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: Ideally, the woofer should have a series inductor for HF filtering but in some applications it's not put. So I didn't illustrate it.

The series capacitor to the tweeter works as a blocker for LF signals, and the tweeter and the capacitor form a simple high-pass filter. Hence the name crossover. If the impedance of the tweeter is 8 Ohms and the series capacitor is 3.3 μF then the crossover frequency will be around 6 kHz. This means that almost anything above 6 kHz will go to the tweeter.

Could it be that adding the tweeter changes the impedance (or some other parameter) that the head unit sees and it changes equalization in response?

Unlikely. For low frequencies (i.e. below 500 Hz) the tweeter arrangement will have a relatively high impedance and therefore will not affect the net impedance of the load network (e.g. 4 Ohms in parallel with 500 Ohms is still around 4 Ohms).

To illustrate what I hear: I usually had this radio tuned with the treble adjustment to the max (And it was never enough, so I added the tweeters) and the bass adjusted about 3 notches bellow max, somewhat 5 notches over it's neutral point. After adding the tweeters I use treble one or two notches up and bass about three notches up and still think I hearing louder bass than before.

Normally, in sound engineering, above 3 kHz is considered as hi-mid and, in some cases, treble. So a crossover frequency of 6 kHz might be somewhat insufficient for your application (Note that if the speaker impedance is lower then the crossover will go further. e.g. 12 kHz for 4 Ohms. 8 Ohms of impedance is just an assumption here). For almost all genres, almost all music recordings have higher low frequency (below 500 Hz) energy compared to mids (500 Hz - 2-3 kHz) and trebs (2-3 kHz and above). So, with a crossover frequency of 6 kHz, you might be missing a very large amount of the HF content.

In conclusion,

• a crossover frequency of 6 kHz for a tweeter is a bit high. Adding capacitors in parallel will reduce the crossover frequency but we don't have any info about the tweeter speaker's response on its own. The speaker itself might not have a good response at lower frequencies (e.g. 3 kHz), or even worse, the tweeter speaker (or the entire module) might be "dodgy".
• The series inductor in woofer module might be missing. Remember that the series inductor should be placed to block high frequencies so that the woofer does not try to produce HF content (those speaker are generally terrible at those frequencies but who knows).

so my question is whether adding these simple items in parallel can somehow affect the low frequencies too?

Unlikely.

I guess the capacitor and tweeter offer another current path for high frequency signal, but low freq. should remain unchanged, or is it?.

This is simply called "crossover". The simplest construction is to connect a tweeter with its series capacitor to woofer in parallel:

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: Ideally, the woofer should have a series inductor for HF filtering but in some applications it's not put. So I didn't illustrate it.

The series capacitor to the tweeter works as a blocker for LF signals, and the tweeter and the capacitor form a simple high-pass filter. Hence the name crossover. If the impedance of the tweeter is 8 Ohms and the series capacitor is 3.3 μF then the crossover frequency will be around 6 kHz. This means that almost anything above 6 kHz will go to the tweeter.

Could it be that adding the tweeter changes the impedance (or some other parameter) that the head unit sees and it changes equalization in response?

Unlikely. For low frequencies (i.e. below 1 kHz) the tweeter arrangement will have a relatively high impedance and therefore will not affect the net impedance of the load network (e.g. 4 Ohms in parallel with 500 Ohms is still around 4 Ohms).

To illustrate what I hear: I usually had this radio tuned with the treble adjustment to the max (And it was never enough, so I added the tweeters) and the bass adjusted about 3 notches bellow max, somewhat 5 notches over it's neutral point. After adding the tweeters I use treble one or two notches up and bass about three notches up and still think I hearing louder bass than before.

Normally, in sound engineering, above 3 kHz is considered as hi-mid and, in some cases, treble. So a crossover frequency of 6 kHz is somewhat insufficient for your application (Note that if the speaker impedance is lower then the crossover will go further. e.g. 12 kHz for 4 Ohms. 8 Ohms of impedance is just an assumption here).

Woofers have larger cone diameter and thus they push the air easily. So the bass frequencies can be heard easily by their nature. And also, for almost all genres, almost all music recordings have higher low frequency (below 500 Hz) energy compared to mids (500 Hz - 2-3 kHz) and trebs (2-3 kHz and above).

In conclusion,

• a crossover frequency of 6 kHz for a tweeter is a bit high. Adding capacitors in parallel may help reducing the crossover frequency.
• we don't have any info about the tweeter speaker's response on its own. The speaker itself might not have a good response or even it might be "dodgy".

so my question is whether adding these simple items in parallel can somehow affect the low frequencies too?

Unlikely.

I guess the capacitor and tweeter offer another current path for high frequency signal, but low freq. should remain unchanged, or is it?.

This is simply called "crossover". The simplest construction is to connect a tweeter with its series capacitor to woofer in parallel:

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: Ideally, the woofer should have a series inductor for HF filtering but in some applications it's not put. So I didn't illustrate it.

The series capacitor to the tweeter works as a blocker for LF signals, and the tweeter and the capacitor form a simple high-pass filter. Hence the name crossover. If the impedance of the tweeter is 8 Ohms and the series capacitor is 3.3 μF then the crossover frequency will be around 6 kHz. This means that almost anything above 6 kHz will go to the tweeter.

Could it be that adding the tweeter changes the impedance (or some other parameter) that the head unit sees and it changes equalization in response?

Unlikely. For low frequencies (i.e. below 500 Hz) the tweeter arrangement will have a relatively high impedance and therefore will not affect the net impedance of the load network (e.g. 4 Ohms in parallel with 500 Ohms is still around 4 Ohms).

To illustrate what I hear: I usually had this radio tuned with the treble adjustment to the max (And it was never enough, so I added the tweeters) and the bass adjusted about 3 notches bellow max, somewhat 5 notches over it's neutral point. After adding the tweeters I use treble one or two notches up and bass about three notches up and still think I hearing louder bass than before.

Normally, in sound engineering, above 3 kHz is considered as hi-mid and, in some cases, treble. So a crossover frequency of 6 kHz is somewhat insufficient for your application (Note that if the speaker impedance is lower then the crossover will go further. e.g. 12 kHz for 4 Ohms. 8 Ohms of impedance is just an assumption here).

Woofers have larger cone diameter and thus they push the air easily. So the bass frequencies can be heard easily by their nature. And also, for almost all genres, almost all music recordings have higher low frequency (below 500 Hz) energy compared to mids (500 Hz - 2-3 kHz) and trebs (2-3 kHz and above).

In conclusion,

• a crossover frequency of 6 kHz for a tweeter is a bit high. Adding capacitors in parallel will reduce the crossover frequency but we don't have any info about the tweeter speaker's response on its own. The speaker itself might not have a good response at lower frequencies (e.g. 3 kHz).
• The series inductor in woofer module might be missing. Remember that the series inductor should be placed to block high frequencies so that the woofer does not try to produce HF content (those speaker are generally terrible at those frequencies but who knows).