Hooray! It was a "ground-loop" problem I was told, by Castle themselves, by an electrician, and by many people on here. I tried lots of different suggestions. Wiring up earth leads to the chassis of the amp and the subwoofer; taking screens off phono connectors; unplugging various suspect components; arieal leads, etc; plugging various compents in the same wall sockets or in different wall sockets; cleaning the pins on all of the plugs; searching the net for special devices to do the job and just wondering where the bloody hell I was supposed to stick them; etc. Then I finally got around to ordering a Mark Grant subwoofer cable, just to seer if it work (I hadn't bought one so far because I hadn't decided which side of the room my sub was going to end up on). And hey presto. All humming has completely dissappeared. "Ground-loop" problem or not, who knows? But it certainly did the trick. Not only that but it also improved the sound. I have cloth ears, not the golden ears of hifi buffs and even I think I can tell a substanital difference in the sound. And simply placing a hand on the sub when it's running confirms that it's far less sluggish (not that it was that sluggish to begin with). For anyone with similar problems, a highly recommmended way to depart with 25 quid before you try all the other alternatives. Even if it doesn't cure the hum, it may well make your sound better plus Mark promises a refund if you are not satisfied with the product.