Thanks for the comments, very helpful. My electrician is excellent and has done lot of work on the house in the past, so I won't have problems with the GFCI stuff or lazy shortcuts or any of that. Having a dedicated breaker box is a nice idea. Amp is a ML No. 332 driving big Magnepans @ 4 ohms - not completely crazy, but still a relatively high current setup. A lighting circuit is already in place so I guess I'll go with the 2 20A circuits, one for the amp and the other for other electronics, another 15A for other outlets (no huge subs are in the future) and maybe put the lights on the same box. I got isolation putty for the outlets. As for hospital outlets, my theory is that the tighter contact has to be a good thing and the case material (nylon) is certainly more robust.
Apart from that, isolated walls, rockwool etc are all part of the plan. The space has a dedicated duct going straight to the HVAC unit and no other pesky ducts to worry about, so minimal ducting, ventilation or sound transmission issues. Egress is though 2 doors separated by a hallway. If that proves to be too loud then I'll soundproof the doors.
[sidebar: I have a powder room on the side of a large kitchen/'great room." The arrangement and construction was totally oblivious to noise issues. It was remodeled with insulation, better sealing of everything and a completely sealed door. Now, standing right in front of the door, the toilette is inaudible. The Wife loves it. Nice!] Cheers,