in your SVP-Pro??? Or any other pre for that matter?? I just loaded my SVP-Pro with 4-Electro Harmonix 12ax7's and a Jan 12au7. When I got it, it had 4-Sovtek 12ax7's and an Electro Harmonix 12au7. The difference in tone was substantial. With the Sovteks and EH, it seemed to be more crispy on the top end and lacked some punch and low end, but I also had horrible problems with microphonics in all 5 tubes, hence the main reason for the re-tube. After replacing them with the EH 12ax7's and Jan 12au7, it really warmed up the sound and gave it a more old school type of vibe, which I like. The top is more smooth now and it has alot more low end and punch now. I have a SR5, so the added low end in the pre helped to balance out the sound when eq'ing. It was weird with the new sound at first, but I'm starting to dig the sound and they have a good grind sound when I crank up the Drive knob. I think once they get a little more broken in, they'll sound even better. I just put them in on saturday, so they've only got like 6 hrs on 'em so far. Let us know what kind of tubes you've got in yours and what kind of tone you're getting.