An electrician could run a separate spur from the main breaker board to the sockets the AV kit is specifically plugged into.
I think the key is to separate the spur they're on from all the other electrically noisy items plugged into the same ring.
Have you tried turning off the fridge/freezer, washing machine, dishwasher, cooker, etc to see if the intereference is generated from within your own house? If so, I think you may have an answer as to whether a separate mains spur is worth it.
When I put my new kitchen in I had 'leckys' in and they wanted an extra £200 to install a separate spur to the living room AV sockets, pull through speaker cables in the walls AND install speaker wall sockets for the side and rear surrounds.
Well worth the money, but even prior to this, I'd never had serious hum problems (with the exception of the BFD) so I can't claim it actually soved anything, but it certainly hasn't hurt.
I'd try disconnecting the earth from the sub just to see if there's any mileage in geting a separate spur installed though. It'll certainly be cheaper than throwing cash at that shiester Russ Andrews!
If you don't fancy any of the above, it may be worth considering 'home brew' mains leads based around ferite cores, like those on
TNT Audio.
I know a reviewer on that site who suffered mains interference and solved his problems with the Merlino.
Russell