Tube progression, so far....
Stock t7-flat sounding, rolled off top and bottom lack depth and width of soundstage
RCA x7a ribbed gray plate-better than stock
RT u7wa-ribbed gray plate-better than the RCAs but one was a bit noisy (couldn't stand it, wish I could have listened longer)
Brimar u7wa-gray ribbed long plate-better than any above-wider soundstage, increase in bass
Sylvania JHS 5751-wow, a direct hit, improvement across the board-Very nice soundstage width and depth, plenty of detail and prat! Gain match with volume control into my Mc DNA-125 is right on.
Mullard x7-short plates, halo getter-A MAJOR DISAPPOINTMENT-detailed but very flat-lost the soundstage completely (I hope everyone does not read this thread because these will be for sale soon!)
GT ax7m-playing now, just got them today-These are nice, very, very close to the 5751s mentioned above. They may lack some of the fine detail of the 5751s but not much. I can live with either.
I would like to try some nos 50s long plate Mullards or Telefunkens but I got to agree with Swampwalker, it seems a bit crazy to put $100 plus tubes in this pre (or does it?). For $50 a pair or so the 5751s or the GTs would seem hard to beat.
I have never fooled with cryoed tubes so I would be interested to get Stratosaur's impressions...