Hi guys, yesterday i pulled a couple of groove tubes 12ax7M (mullard replicas) out of my marshall head, and replaced them with a fine couple of real Mullard 12ax7, almost new that i bought from ebay for a very cheap price. Well, I already put a couple of Siemens in the power stage, and could appreciated the results if compared with stock tubes. Yesterday i could once more appreciate an impressive difference in the sound. I put one Mullard in the first position and one as phase inverter before the power stage, and suddenly my round, silky, thick sound became rounder, even more silky and thicker. What a change! I would not say it's better, just it's different (incidentally, it is better for me). So this brings a consideration along on which i'd like everyone could say his opinion: mine is that tubes are almost 50% of the sound in a tube amp, and, even for past experiences i had, i say that a poor sounding amp can have great benefits from a tube replacement, given that you have selected and fine tubes available. What do you think?