I've never been, but from the research I've done the only 2 sure thing AMP's in SD are Pine and Coco both located quite a bit north of downtown.

Bayside Spa hands down best in area--close to gaslamp (5-10 minutes)--vichy table shower which is great, nice and clean building and rooms, discreet, parking can be a challenge depending on when you go because of neighborhood/street parking nearby, reasonable price--$60 for first time visitor + tip for girl