I'm wondering if anyone can help me select some preamp tubes for my 400+,any experience with different makes of 12ax7s',and the different charactaristics of each??? I'm after a very warm tone with clean trebley highs and grunting high mids.I find the original mesa tubes (2 yrs old) have a very harsh treble attack (I play with a pick) almost like a slight treble boost overdrives the tubes too much,but if I back off on the treble I loose the high end crispness I'm after. thanks. p.s. the poweramp tubes are about 4 months old.