Audiophile Receptacle question

This should be the last thing you worry about. Isolation from other sound sources, and room acoustics should come first.

Well, since you've stated that you're going to convert your garage into a listening space, it does make sense that before you put up your walls, installing decent romex and receptacles would be one of the first steps you'd take, since it's all exposed down to the studs.

I don't have the experience you're seeking but others will chime in soon enough.

All the best,
Nonoise

It actually is the last thing. Acoustic treatment and sound proofing is all being done and meticulously planned. 

Yes the outlets will make a difference on your guitar amps. Same as anything else. Only thing to keep in mind, since the guitar amp is essentially mono you obviously won't notice imaging or depth like you will with the stereo. But in terms of attack, harmonic development, decay, dynamics, sure. The only thing that would give me pause is the range of what sounds good with electric guitar is so all over the map you may actually prefer the grainier more distorted sound of a stock outlet. Oh well. When you're looking at building a whole room I doubt you will balk at swapping a few outlets.

As for which? Synergistic. Totally.

I would go with hospital grade receptacles (Hubbell, Pass Seymour) and save a lot of money to get the same results. The objective is to cover and grip the power cord prongs as tight as possible and that's what HG receptacles do.

But I think vintage guitar amps will do a lot better with a thicker gauge power cord. I remember plugging my Twin Reverb into a cheap power strip/extension and it sounded as if a blanket was placed over it.