Question for tube aficionados

I have a vintage Conrad-Johnson PV 11 preamp.
 I’ve always re-tubed it  with the cheapest tubes from Tube Depot (e.g. Electro-Harmonix 12AU7 and 12AX7, Sovtek 5751 and the like.). I’ve been very happy with the sound.
 My question is:
 Would it be worthwhile to spend more money on more exotic tubes?
  Would there be any appreciable difference in the sound?
 I’m powering the system with NuForce Reference 9 V3 monoblocks.
Nos vintage tubes sound significantly better than any tube made today and they last much longer. I use Mullards, telefunken and rca 12—7’s. The rca clear tops are nice and about 12$ a tube. I don’t like nor have had good luck with modern day reissues. 
You can try swapping tubes for sure but even better is to change a few of the resistors.  Carbon film, metal, shinkoh, caddock, etc.  cheaper way to go and a lot more fun!

Happy Listening.
Yes try NOS OS 5751s they were all the rage 10 years ago.  They are more reasonable these days.  I advise you get Sylvania blackplates, RCA easier to get and even GE again blackplate and their 5 star brand.  If your up to a search the Raytheon windmill getter 5751 is very good,  Get triple mica preferred over doubles.  Sylvania's Gold Brand and RCA Command series are worth looking for. Good Luck tube rolling is a slippery can be addictive, PM me for more advice/help.
I agree with previous comments to first replace the 12AU7’s. But, substituting Gold Lion 12AU7’s will be a horizontal move; there will not be a dramatic improvement. To really open up your preamp you need to go with NOS tubes. There is so much good info on the sonic signature of NOS tubes at Joe’s Tube Lore...
and Brent Jessee’s site...

And almarg has stated an important point about the use of the 5751 variant in your CJ preamp...
Please note: The indiscriminate substitution of type 12AX7 tubes for the 5751 will seriously degrade the sonic performance of the unit.

The 5751 has lower gain, so it's recommended to stay with a 12AX7.


Based on the recommendation at Joes Tube Lore, I’m getting a pair of RCA 12 AU7 in place of the Gold Lions.  According to Joe, they should cost around $5.00. Tube Depot has them for 12x that  price, but it will cost me about the same as the credit I’m getting.