Need Help Combating Power Amp hum

I have a vintage Forte Model 5 power amp purchased used on Audiogon. It is a nice sounding amp, but is plagued with a constant, low level HUMMM which is just audible from my listening chair when the room is very quiet. I understand hum modulates/distorts the music which is undesirable.
My speakers are an MTM design with 90dB sensitivity (the other owner used Magnapans and didn't notice the hum).

The amp uses a toroidal power transformer and all the signal wires are routed carefully away from the power supply components. The hum occurs even when the inputs are shorted/disconnected, ruling out my other components. Power cord polarity and grounding were also checked.

Any ideas/techchniques to eliminate this hum?
Do you have a cable tv cable or powered sub connected to this system if yes you need a Jensen isolation transformer
Most hum is caused by a ground loop created when you have two or more components plugged into your electrical service with third prong grounds. You can try to cheat out the grounds on everything other than one component (usually the preamp), which gives your system a single ground path to your electrical service. Cheater plugs are available at any hardware store.

As stated in a previous post, cable TV connected to anything in the system is also a typical cause and can be solved with an isolation transformer.
It hums all by itself... As already mentioned
I'm listening to just the power amp w/everything else
Hi Chasmo
I had a somewhat similar problem. This sometimes happens when the AC power line is not 100% grounded tho that wasnt my issue. When the AC is not 100% grounded, your speakers will probably pick up the 50/60hz AC frequency which you are hearing as a humm. My problem got solved by using a balanced interconnect between my pre and power (everything else is single ended) - dont recall if the Forte 5 has a balanced option. I also inserted a transformer based power conditioner which helped make the background quieter.
Best regards
The hum occurs even when the power amp is the ONLY component used (i.e. everything else was unplugged) it appears to be within the power amp itself. No balanced options in my system, as all components use single-ended RCA.
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.
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.
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.
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!
I had a similar problem with several Amps. I could hear the toroidals from across the room. I now use a PS-Audio HUM buster, everthing is now quiet. Most likely I had some rogue DC on my AC line.
Sorry Chasmo, when I read your original post and you mentioned speakers, I thought you were hearing hum through them. Sounds like transformer hum.
Opps, me too. If you are hearing it entirely from the amp body, I would open it up and see if the toroid is loose. If not, loosen it and see if there is rubber beneath it. If not, add three small strips and retighten.
Normally I have to be within 3ft of the speakers to hear the hum, but during late-night CRITICAL listening (no traffic or neighborhood sounds and I turn off the fridge in the next room!) I can JUST detect it from my listening seat 8 ft away.
(this line didn't come through on my last post).
Take to a tech unless Forte has a better idea.
Found the problem!
A tech friend tightened up the chassis ground screw and
the hum is gone.

Thank, Guys.