RFI from steel TV bracket?

But when I unplug the input to the amp (with the output still connected to the preamp)......the hum disappears

I still think there is a possibility that the interconnects that go from the preamp to the amp my not be shielded enough for the new environment. Try a different pair (if you haven't already) between these two (amp and preamp) that may have better shielding, or a different design. Last resort (don't laugh), even some budget ones from Radio Shack, or even a big box store just temporarily to see if this can be the problem.

Hifihvn makes a good suggestion. You might be surprised how good some of Radio Shack's cables actually are.

Now. Since you are running single-ended cables, if there is a hum source (like the primitive transformer thing I mentioned), even if the cables are shielded you will still get a hum as single-ended cables have no 'Common mode rejection'. (IOW if you have balanced cables you may not have noticed a problem.)

So what I would do is temporarily reroute your cables so they are no-where near that bracket and for that matter, any power cords too. If you have to shut down everything but the amps, go ahead a do so for the purposes of this test. So hopefully you won't have to go to Radio Shack :)

Halcro, you may want to look at the Mogami 2534 or 2549 cable. It's what allot of studio's use because of their abillity to be quiet. Low capacitance and fairly cheap to what's out there.

http://www.markertek.com/Cables/Audio-Cables/RCA-to-RCA-Cables/TecNec-Cables-Connectors/MSC1-5RR.xhtml

http://www.mogamicable.com/category/bulk/microphone/quad/

http://www.mogamicable.com/category/bulk/microphone/quality_balanced/

Brad

I still think there is a possibility that the interconnects that go from the preamp to the amp my not be shielded enough for the new environment.

That was my initial thought which is why I wrapped them in aluminium foil which reduced the hum somewhat.
I hope we're all right as I've just ordered an XLR pair of Cardas Clear interconnects and George Cardas tells me they are the best shielded cables he knows of?
He guarantees my satisfaction.......which is a great attitude from a manufacturer.
I'll know in about 3-4 weeks when I receive them. Fingers crossed :^)
Thanks for all your help.
Cheers
Henry

Received the Cardas Clear XLR interconnects today and installed them between the Halcro DM10 pre and the DM58 monoblocks.
Absolutely no hum or buzz whatsoever.
The shielding in these cables is obviously so superior to the old Hexlink 5 interconnects and/or the balanced XLR method of connection combine to lower the noise floor from the speakers to nil :^)
Now I need time to see if I can 'hear' other benefits?