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.
