Just wondering if anyone out there has spent the money, or had enough of them laying around, to put either the older brand name tubes, or else new gain matched tubes in their SVT 4 preamp? If you have, were there large sonic differences? I'm about to do the retube and wonder if it would make any real difference at all. Money is not too large an issue here, "if" I'd be able to appreciate anything as the general listener because of the upgrade. I know every valve has different gain characteristics (slight) and want someone more experienced to tell me if I should spend $38 or $200+ on three 12ax7s.