Good one!
Testing if a cap is good or not when used in front of the input stage of an amplifier is quite easy:
- 1) Get/borrow a dual spot oscilloscope.
- 2) Get/borrow a square-wave signal generator (most scopes are already having built-in generators). Do not use a DAC as a square-wave generator, it will not work properly!
- 3) Feed the amp with 1V RMS square-waves at 20Hz and measure the output of the amplifier with your scope (1...2V RMS on outputs should do). On the same screen measure the output of the signal generator. Do a print screen or photo of both spots plotted on the screen. The two plots will probably not superimpose due to the very low 20Hz used, unless input caps are having a huge value or the amplifier is DC-coupled (but this test makes no sense now).
- 4) Do the above measurements for 50Hz and 100Hz. The two in/out plots should superimpose, at least for the 100Hz; otherwise the cap is having a lower value than needed or is of a questionable quality.
- 5) Swap the input capacitor with an "audiophile" one and do the above steps again. Now compare the print screens and check how the square-waves look like for both capacitors.
- 6) Optionally step: short-circuit the input capacitor, you can use a short wire or an isolated clipper or simply do it with a screw driver directly on the back of the PCB. Just don't use any headphones or speakers connected on outputs; use resistors instead, if needed.
I doubt there will be any difference between a regular WIMA MKP capacitor and an audiophile one when used on amplifier's input.