Here's a good test tintin...
Listen to the hifi at usual volume in usual sort of surroundings with lights on kettle on the boil may be even washing machine on.
Then go round the house and tun off as many things as you can and see if you can notice a difference,especially on vinyl.If you can then a filter may just be the thing.But before buying one,check your earthing around the home.Check you have all the earth bonding on taps/pipes etc.This could be an easy fix for a less eggs and bacon sort of time.Some say its best to leave equipment on all the time as the equipment takes time to settle down,but I cant get my head around the fire risk(small as it maybe).
Maybe good shielded power leads might make a difference.I tried these but highlighted other problems with the system.So I went back to "in the box" power cables until I ever get the urge to spend some real money.
One thing I did think improved things are them half round bits of rubber stuff for feet.There are some that say wood feet are better because they dont keep energy stored in them as the rubber ones do,but I had good results with them,even on CDs.
http://www.ebay.co.uk/itm/8x-SORBOTHANE ... 288wt_1163
Sorry if you are aware of all this,but there may be others reading who havent come across dirty mains before or realise why their cds sound great at low volume but at proper listening volume they sound poor