The one I use on my bench is a Sencore PR57, an older unit that might have been used by TV repairmen.  It combines a variac, isolation transformer, meter, and leakage detector. PR57

I've used a little transformer like the Triad N68X for old series-string amps and radios with no line transformer.  You could put it in a box for low power applications.  N68X

There must be nice rackmount isolation transformers made for pro audio or industrial use, but since I'm just a fixit guy and experimenter I've never investigated them.  I'd guess you already you know that not all power conditioners include isolation transformers- same for DI boxes.

Maybe a computer UPS/battery backup could be used as a test device.  Unplug the UPS and power the audio device from it.  But the UPS might not provide a clean sine wave output. 

It seems like AC house wiring would be simple, but if you Google GROUNDING VS BONDING, you'll learn all kinds of things that can go wrong.  Soon you won't want to plug anything in at all!

I don't trust the cheap crappy "wall wart" power supplies we get these days.  Sometimes using a different wall wart or a bench DC supply will clean things up.

-Steve W.