I also noticed that the 6SN7 tubes used as power tubes don't even make the LD MKIII break out in a sweat - runs really cool, which I think is good.
In the never ending quest for audio truth I have being trying out different combinations of octal tubes. I am trying to the answer the question:
Do power tubes matter?
The answer is yes. I decided to use some of the poorest sounding 6SN7 tubes I have (based on using them as driver tubes). Put in a near pair of two GE 1950's tubes, and used a good performing Tung Sol 6SN7GTA as driver. This TS tube is a real midget with a short glass envelope. Although I prefer a tall tube as driver I decided not to discriminate and give the little guy a chance.
The main problem with the sound was the bass. Flabby, muddy, without good control of the low frequencies and a dull almost lifeless presentation.
So my conclusion, which is also based on comparing the 6N6P family tubes as power tubes, is that the main difference with different power tubes is the control of the bass and the quickness and slam of the presentation. I am not stating this as an absolute, but at this point I am inclined to think that this may be correct IMHO. (If you think otherwise, I am interested to know.)
Could not stand the GEs, so I popped in a near pair of '53 and '55 RCA 6SN7 tubes. This TS/RCA combo is OK with the only complaint too much sizzle in the treble. Maybe it will go away after using the tubes for a while; you'll never know with these used bargain tubes. With a NOS tube you can count on it changing for the better after burn in, but here I just don't know.
Well, I found out. LOL; even after extended playing the treble was too sharp. Put in the tried and true RCA heater wire on top of mica tubes, and popped in the Japanese Channel Master as driver, and all is well again.