What are the best tubes to use with the Ampeg SVP Pro preamp? It has some real cheap-o's in there from the factory and I notice a slight bit of noise. My SVP uses four Sovtek 12ax7wa's and one unmarked 12au7. I'd like to hear suggestions on personal experiences with different tubes in this preamp. Thanks, Mike S p.s. I play rock, with little to no use for very high frequencies (no slap, etc). This makes it hard to read reviews for tubes aimed at g***** players.