I have this amp and some good experience with this issue. I have found that it's not just the transformer that hums. The large Magnecraft relay just to the left of it also hums.
I even tried the PS Audio HumbusterIII on this amp. It's a great product and works on other transformers that are effected by DC, but it doesn't do much of anything for the 200c that I have.
Also, I sent the transformer out for service to be vacuum dipped to tighten things up, which helped just a little.
I investigated the relay further, unhooking all connections to it within the amp. I then connected a power cord to the relay and a power strip to turn it on. Hummmmmm.
The 120v AC coil hums due to alternating current. The positive half of the sine wave closes it, the negative half makes it want to open up. This happens 60 times a second (60 cycle hum). If you can somehow feed the relay itself with direct current, the relay hum does completely cease to exist.
My transformer still hums a little, but with the relay now silent, the whole thing is much much more tolerable now. Even if you could match up a new toroidal (which can obviously still hum...), the chances that you'll find one that actually fits the chassis is highly unlikely imo.
Please note that you could spend big buck$ trying to resolve this issue, or put yourself in some DANGER if you don't have experience to do things on your own. The worst thing is that if either of the main boards fail, which they can and do, are 100% irreplaceable.
You could try Cullen Circuits and see if they can help. (Or sell it off).
