Problem with Counterpoint SA3.1...Hissing/scratchy

I've had this for about 3 years now along with my Dynaco ST-70 with satisfying results. Recently I've been getting scratchy/poppy/hissing sounds that I've narrowed down to the Counterpoint. I've got my Dynaco hooked up to my Outlaw receiver's pre-outs for a temporary solution and no noises are present. When I switch the 6dj8 tubes around in the pre(I'm thinking that if some of the tubes are bad I should still be able to figure out which is good and install that in the phono section BUT... 1)None of the tube sockets are marked as to which stage they drive and 2)Sometimes I get no sound at all after just switching the same type of tubes around) I don't get results. Is this what typically happens when tubes go bad in this pre? What other problems may this be? Should I really shell out $100+ bucks for new tubes or just get something else in the neighborhood of $400-$500? This had some mods done prior to me getting it but I haven't a clue what they are. Any suggestions would be greatly appreciated.