Yes, I agree that it would be best to not install a 20A receptacle on a 15A line.
The alleged sonic benefits of some 20A receptacles, if they exist, would not be due to their higher current rating per se. They would be due to other less well understood and less well accepted factors, which could very conceivably be found in a similarly designed 15A receptacle as well.
Putting in a 20A receptacle would also be a code violation, as has been pointed out. The reason for that code provision, if it is not clear from the previous comments, is to prevent devices that would draw more than the 15 amp capability of the breaker and/or wiring from being plugged in.
The reason I mentioned earlier that it would be a no-no to put in a receptacle rated at less than 15A, if one exists, is of course to eliminate the possibility of the receptacle itself being required by the plugged in devices to pass more current than it is rated for, which could lead to overheating that would not be protected against by the breaker.
Finally, if it is not clear, be aware that there is no direct correlation between the 120W output rating of your amp and its ac current draw. Tube amps and pure class A solid state amps, in particular, are highly inefficient and can draw far more power from the ac line than the power that is put out to the speakers. That said, I agree that you are very unlikely to approach 15A consumption by the system with just about any 120W/channel amp.
Regards,
-- Al
