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.
@rvpiano, the RCA BLACKPLATES are very good tubes for a decent price. Known for their dynamic open sonics and classic tube sound. Make sure the pair are matched and tested for noise.

There are many RCA Blackplates at various prices on Tube Depot.
 Do you have a preference?

Never mind, There’s only one 12AU7 Blackplate there!
My mistake.
 Sorry for the post
I ended up ordering a matched pair of 12AU7 RCA Cleartop (side getter) tubes from Brent Jessee, raved about by both Brent Jessee and Joe’s Tube Lore, for $69/pair.
Tube Depot is way overpriced.  I’ll have to use my credit on something else.
I like the 5814a variant of the RCA Blackplate. It is more dynamic than the 12au7. The Cleartops are good if you like extended highs.

Re... the Blackplate 12AU7, the older the better. Early 1950s, low noise is best for the classic tube sound.