i have an 80's mti ampeg svt. the preamp seems too hot. i play a passive p bass. is it because they subbed the 12dw7s for the 12ax7s? is the mod expensive to change 12ax7s to 12dw7s? i've heard its only a few resisters. ive played 70's svt's and they just sound better, must be the 12dw7s. OR maybe its time for a cap job.lol.