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.
I never had good luck with that variant. They often sounded harsh and microphonic. So I'm not the best person to ask...
I contacted Conrad-Johnson.  They sell a complete set of recommended tubes for the PV 11 for $310.
I wonder if I should go in this direction.
The Conrad-Johnson description of the tubes is as follows:
 “V1 & V2: 12AX7 MULLARD| V3: 5751ELECTRO HARMONIX | V4, V5 & V6: 12AU7 YUGOSLAVIA.”

No further description than that.
It sounds like these are new production tubes. To go in this direction would get u right back where you started.
The only upgrade path is to try NOS. 
I think lowrider57 may be incorrect. The Mullards could be new production or old. The Yugoslavia 12AU 7's are definitely old stock. They haven't been inproduction for 20+ years. When this unit was first produced I don't think  new production Mullards were available - my guess is that they are more probably old stock and I think that the NOS and new production do not have the same tonal qualities. 

I'm not familar with your pre-amp but I suspect that the V1 and V2 positions are likely the positions most affected by tube changes. If that is so, and if I wanted to play with tubes changes, I would start there keeping in mind that different brands have different tonal qualities and it helps a lot in selecting tubes you want to try to have some 'tonal' goal and try tubes that have a similar tonal signature.