I have a pair of 20 years old electrostatic speakers that have an analog first order high-pass filter before the electrostatic panels, so there is a capacitor in the audio signal path. The original capacitors were electrolytic (I suppose the capacitors were about the same age as the speakers) with a capacitance of 100 uF which I replaced (by recommendation) with Mundorf MCap 250 MKP capacitors. After the switch I subjectively heard an improvement in the sound quality with a more refined upper end. I should note that I do use a calibration system with mic that should fix most big problems in the signal frequency response.
My question is if it just was an placebo effect or if there is actually some measurable quality in the capacitors that can cause the improvement in sound/signal quality? If so, what causes this and is it a consequence of the aging of the capacitors or have the type of capacitor a measurable effect on sound/signal quality (there are very expensive "high-end" capacitors sold by DIY speaker stores)? Does the same differences exist with different types of resistors that are in the signal path?