Why do NOS Tubes sound better than new tubes?

Mr X- Actually; it depends on semantics. Read the first paragraph of this article: (http://www.svvintageamps.com/tubebias.php) Most often; people refer to, "over biased" as being too hot, rather than the other way around. I simply covered both bases(sorry for the confusion*). Of course; running a tube slightly cooler than it's design parameter, will extend it's service life(as with virtually any other electronic component). Happy listening! *Having reread my earlier post: I did say, "either of those conditions" which was a brain-fart on my part!

Rodman - Thanks I was starting to worry about the slight under bias. I know tubes won't last as long as transistors and want to get the most life out of my tubes. As I've stated overall I prefer NOS (EAT is the only exception and maybe the Shuguang Treasure Z or Psvane). Most of the NOS and A(lmost)NOS I have are very close to EAT Kt88 prices. Thanks again for clarifying the under bias - I'll sleep better tonight LOL.

I need to sleep better AND stop posting at 3:00am! Again: Sorry for the anxious moments.