You know what's coming don't you?
SHUT UP AND PLAY!!! 
But seriously, I truly wonder if most of us would even hear a difference.
Say you have four different type caps. They are all measured and for this purpose all have the same value.
The voltage ratings are all the same.
In fact, every measurable value would ideally be the same or within 5%.
From the cheapest Chinese $.02 film cap to the most expensive $25 audiophile film cap.
For this test I think it would be best to limit it to five caps, because after a while I think your brain would try to work too hard to hear differences.
Then they are put on a rotary switch installed in a box so your hands don't effect the capacitance and short leads are ran inside the amp to say, an interstage coupler between V1 and V2 of an amp unfamiliar to you, or some other agreed upon spot in the circuit but I think a coupling cap is a good spot.
The amp should be a high quality amp and preferably not a pcb amp, don't want to open that can of worms up...
Now you obscure the amp from view and face the loudspeaker(s).
You are playing a guitar of your own and know what to expect from it.
The amp volume is turned up but not uncomfortably loud.
The person sitting with the amp turns the dial and gives you a nod.
He may or may not actually turn it. But records your opinion of each nod.
I really wonder if there would be that many audible differences.
I would be totally interested in the results.
Because on amp forums, you will hear usually one or two of the guys that seem to have a lot of experience tweaking amp tone exclaim the virtues of Cap A or Cap B with flowery adjectives that lead you to believe they are the best thing since sliced bread.
Hey, they block DC and pass AC.
But how they sound while doing that is what we're after.
Did anybody happen to see this?
http://acapella.harmony-central.com/showthread.php?t=2232581
