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.
Thanks to everyone for their suggestions.

My speakers are modified Dalquist DQ 20’s.

Overall, the system is fairly well balanced.  What could be improved is occasional brightness in the treble, especially in the line stage.

 Ideally, I’d like to replace all six tubes, depending on the cost.
Vintage pre-1970 GE, RCA, Sylvania, Mullard and Telefunken are still the best made and sounding of the 12a*7 types! Far better (and longer lasting) than present day production! That is why they command high prices!
Cheap present day 12a*7 types will never equal the sound quality and lifespan of vintage types!