i've got a tube-driven overdrive pedal ( Hughes & kettner tubefactor ) which has one 12AX7 in it.. Some guy at another forum mentioned that putting a 12AT7 in it would make it sound better with a bass.. But.. then another guy said that a 12AW7 would sound even better.. this stuff is so confusing.. what's the difference between the 12AX7, 12AT7, 12AW7, 12AU7, etc ?? and.. what tubes are reccomended ? i've asked my local guitarshop and they can get Fenders, Groove Tubes, Marshall's, USA sovtek's ( $$ !! ) and russian Sovtek's.. c'mon guys.. give me a quick-tutor on tubes here !