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.
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.
@newbee , you may be right. I traced the PV11 preamp back to at least 1992, so the Ei and Mullard could have been NOS. But CJ is now re-tubing the unit, and $310 is a very low price to include 2 NOS Mullards.
+1 lowrider57. I’m not suggesting that the CJ recommended tube option is a bad one. For that cost I’d would pursue the NOS tube pathway. I like the idea of trying the RCA tubes as earlier considered above. I’d have CJ clarify if their tubes are NOS or current production. Again their choices may be very good sounding. Truth is there are many good tube rolling options for the 12AX7 and 12AU7 family.