Hey Howard- I believe the 2A3 tubes in those are direct heated triodes (someone correct me here if I'm wrong). Combine that with a 115 db efficiency rating of your horn speakers and you are bound to have some hum at idle. Given the smaller size of the room they're in you are probably more aware of this. Since I know you are speculating on a larger space to put these in, perhaps you may want to wait to make that decision. Given that there is no ground loop problem, you can try a power conditioner, but I'm not sure that will help if it's the direct heated tubes causing it. Some of that may be the way the circuit is designed as well. Did you phone George and ask him? He'd be the one to ask........since they're on a 30-day trial period can't you just return them if you are not happy with them? FYI my 300B's also have audible hum through my LaScala's. Just the nature of the beast. Doesn't bother me at all though. I left my computer on at work and it's automatically collecting my emails, so I'm not getting them at home right now in case you tried to email me about this.
Best,
Marco
Best,
Marco