the NOS mullards indeed tame the beast, overtame it in some cases. I put at7s in as driver tubes in my amps (6550s) and everthing rocked. Could have been as much that my power tubes that were also replaced were junk but the at7s still seem pretty lively. Put at7s and ax7s (at $100 ea for the ax7 which were matched) in the preamp and details as to space and instrument seperation popped out everywhere, cymbles sounded great all the rock and roll just up and left. Polite would now describe my system. For some things it's a vast improvement but for others not so much if you just want to bang your head.
My guess is that the mullard ax7s have a little less gain and the matching may add a level of smoothness not present in lesser tubes. Percieved volume level is down which indicates greater smoothness in the highs and mids as well