As i said in my post i think its either a earth problem or mains spikes.
All mains have certain "noise" but some are worse than others depending on numerous factors.A mains filter, in my opinion would work.get get from maplins, comet etc etc.
also is the sub placed to near the tv or other devices.
http://www.mainsfilters.co.uk/
Here is a similar unit.
http://www.maplin.co.uk/Module.aspx?ModuleNo=46830