Bypassing Caps - what is it really about?

https://components101.com/articles/decoupling-capacitor-vs-bypass-capacitors-working-and-applications#:~:text=The%20Bypass%20capacitor%20is%20used,spikes%20on%20the%20supply%20lines.

The main capacitors, or AKA "filter capacitors" in an amplfiier will together with the power transformer(s) influence sound quality overall, but most importantly have great impact on the level of clarity and transient response in fast-paced music; also important for resolving nuanced details in slower tracks.

So I’m not talking about bypass circuits. I’m taking about what hi-fi people call "bypassing a capacitor", which is running two capacitors in parallel, typically using a small value to piggy back the main value.

Larger capacitors act as inductors at AC frequencies. The larger the cap and the higher the frequency, the higher the "resistance" presented by the cap. This can impede or roll off frequency response (impedence). A small value cap across the large value cap allows those frequencies to pass. Kinda like overtaking a slower semi on a small road.

Larger capacitors tend to have higher inductance and dielectric absorption affecting higher frequencies the most.  Bypassing with smaller capacitor (creating alternative path at high frequency) improves overall performance.  It also allows to use cheaper main capacitor for cost reduction.  Huge electrolytic caps (with higher inductance)  in power supply seems to be ideal target for bypassing (speaker current closes thru them), but low ESR bypassing caps in parallel with inductance of main caps can create parallel resonant circuit.  Using many filter capacitors in parallel reduces overall inductance.  There are better, lower inductance "Slit Foil"  BHC (now KEMET) electrolytic caps, but they only go to about 80V and are expensive. 

noromance has it more or less. Bypassing caps has it's basis in good EE design, especially older but also best bang for the buck designs. 

Think of actual caps as a series of parts:

--> Capacitor --> Inductor --> Resistor ---> 

As the frequency goes up the cap no longer acts like a perfect cap and the inductance becomes a problem.  By bypassing this with the appropriate values of smaller, higher quality caps you can get maximum value for your money.  Imagine a situation when you need 100uF.  The cheapest option may be an electrolytic but may become inductive too soon.  So you buy a 100uF and bypass it with something pricier (per uF) but smaller, like a 2uF film or tantalum cap to overcome the internal inductance. 

The reality of audio gear however is complicated.  Sometimes a good bypass cap can help, sometimes it does nothing.  I like Clarity CMR caps a great deal, but at values larger than 4uF or so they do audibly benefit from a small bypass cap.  Below this value I've not found a bypass cap to help at all.  

I suggest you get a pair of Audyn TruCopper 0.1 uF film caps and experiment for yourself.  The argument of the value of bypassing or not is not ever going to be solved in an online forum. :)