Mike P said:
If your $2000.00 amp has a brittle/thin/piercing top end, there are other issues at hand.
Well, first off, I wouldn't purchase a $2000 amp with a brittle/thin/piercing top end. Secondly, I wouldn't drop $2,000 grand in hopes that a $9.95 tube would "fix" the tone. That's ridiculous.
Prove it. Go ahead and record a track with the "offending tube", then record it with the JJ.
There is VERY little difference between the various "cheap" 12ax7 tubes. If you can't dial in a great tone with a set of Ruby's or Groove Tubes or whatever, the issue isn't the tubes.
I've got a Blackface that has some of those piercing highs, where trying to dial it out takes away too much top end
If I throw in a Ruby or (GASP!) JJ I have lying around, it takes off that top end while keeping clarity
I've got an SL+ (old values) that lacks some mids, if I throw in a Tung sol it adds mids but also too much sizzle,
if I throw in a JJ it also adds mids but turns the bottom to mud
grabbing that ruby it adds mids while keeping low and high end pretty much the same
I've got an Ultra lead where I can't 'up' it's presence, no matter what tube I throw in that
It remains smokey, and the only way to change it is using an added EQ
The characteristics of it's sound are too great to be changed by tubes (or guitar p'ups for that matter)
So the Blackface and SL+ 'improve' with certain tubes, they didn't sound bad to start with, but IMO the change made them sound better
The UL is where you are 100% right, nothing will change that for the better
but the other "tone issues" were -to my ears- easily corrected
There still is a difference between 'issues' and 'sounds like ****'