Need Help Combating Power Amp hum

I sold a dead quiet amp to a member. He had this problem. I just knew it wasn't the amp. About 2 months later he found the problem--a fluorescent fixture.

Interesting, but no fluorescents involved. When I listen I usually turn off all the lights - still problem with hum.
BTW audio system is on dedicated line and no hum when substituting a different power amp.

Well, the easiest answer is to take it to a tech. I don't think your speakers are so efficient that they are revealing the problem. Obviously it is not a ground loop problem. Your amp may be picking up something being broadcast from nearby, but you say there is nothing nearby so I think you need to turn to the design of the amp or a part that has failed. Have you talked with Forte about your problem. That would be a first place to start.

If the hum is present when nothing is connected to the amp inputs then it is probably not a ground loop. But, as Davemitchell mentioned above, try a cheater plug plug to remove that from the beginning. If the hum continues with the grounding pin out of the picture using the cheater plug, then I would think that the source is either dirty a/c (like fluorescent lighting as mentioned, as dimmer swith in the circuit, etc.) or intrinsic to the amp. 90db is not so sensitive that an internal hum would be audible based on sensitivity of speakers alone, plus you should have noted it with your previous amp.

When I built an dedicated listening room onto my house a few years back, I had dedicated 20 amp circuits with one outlet (cryo'd Albert Porter's) per circuit run for each amp and one 220v for my power conditioner for all the other gear. When I installed everything there was a tremendous hum coming through my speakers. Nothing was different or new except the room and it's wiring. One by one I removed components from the chain and it was when only my pre/pro was connected to the amps the hum appeared, in all channels. I thought that I could just put a cheater plug in the preamp and that would fix it, but it did not. Simply connecting the pre to the amps caused the hum. I eventually replaced all my power cords with PS Audio because I could unscrew the grounding prong from them. I've learned that even though the circuits have a common ground, voltage potential along the circuits' path may vary enough to induce a hum when the circuits are connected, i.e. when the two amps and the preamp are connected, they have three separate grounds which did cause a hum. The source components, plugged into the same conditioner as the preamp, are fine to be grounded, but the preamp and the amps must be ungrounded.

This is just my experience, which I share not because it applies to your current situation, but for others searching in the future.

sounds like you've got toroidal transformer hum - look for my thread titled "best tube pre for bryston 4BSST", where some folks put links to articles about this. I also made myself nuts doing away with it!
I still have some equipment with toroidals, and they are silent. But this one piece drove me CRAZY!