How does one add bass to an audio amplifier? Assuming we have a basic pre amplifier and power amplifier in hand, and we have an ordinary speaker at the load. Do we need to buy another specialized speaker for bass since last time I went to an electronics store, they sell tweeters.
You say you want to add bass to the amplifier and then mention speakers. Do you want better response in your entire sound system? When we talk about the amplifier, we are only talking about the power amp and possibly the pre-amp. The speaker is a separate entity.
When we want to add bass to the signal, generally some form of equalization is used before the power amplifier stage. If you want to add bass response to your speaker, you will generally either use a better speaker (usually larger with a better enclosure and better materials), or you can add specialized speakers for each frequency range. Many speakers have multiple drivers in a single device, like this studio monitor:
Notice the smaller and bigger speaker. These speakers separate the frequencies using an internal audio crossover and play certain frequencies from the appropriate driver. If you want you can buy separate sub-woofers for your music, which will play only the lowest frequencies. Depending on the system you use you may need to make a crossover yourself.
Please go get a book on some of this material and try to cement your fundamentals. For the audio specifically, I like Thompson's Understanding Audio. For the circuits, I learned on Sedra and Smith, but there are tons of good books out there. You can learn all this stuff independently if you put your mind to it. Good luck!
Usually you have an "equalizer", which is a set of adjustable filters between the preamp and the power stage, and a "crossover", which is a set of filters between the output and the bass/treble speakers.
Generally the larger the speaker, the better the frequency response at low frequencies. Designing speaker enclosures for best response is an art in itself.