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.
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To your question to Wolf, no you can sub in 6sn7s for 12au7s or 12ax7s

Here's the link to Brent Jessee' website (sells mostly NOS tubes) if you scroll down a bit to the yellow section where you see different tube types listed click on 12ax7 and it will bring up not only his list of available tubes but impressions on various respective brands; you can do likewise for 12au7, etc.  (Note: I am not Brent nor affiliated with his company other than as a customer) - Might be helpful. Also , if you call him , say Andy at Vintage Tube Services and tell them what type sound you are looking for they can steer you accordingly. 
Oh, do try some other tubes than the EH and Sovteks!  They are "adequate", IME, but certainly nothing special.  I have had great success trying 5751's in place of 12AX7's at the driver station for an EAR 890, and there are great options for voicing the amp in a direction you desire, all of which are definitely superior to your current tubes for musical finesse.  I like RCA 5751 Triple Mica Black Plates when I want a richer presentation, a "warmer" influence, as it were.  If you want a more articulate (but potentially leaner) presentation, try a GE 5751 Triple Mica Black Plate.  Now, when you want the best all around performer, IME, try a Sylvania 5751 Triple Mica Black Plate.  That is where I settled, but each amp, system, person is different....

Also, Joe's Tube Lore is definitely worth a read through over at AudioAsylum (in their FAQ's section for tubes).
You’re using pretty decent tubes but as others have pointed out you can definitely move to a higher level for improved sound quality. You’ve been given some reputable tube vendor commendations. I can vouch for Vintage tube services. Much knowledge and very good selections from Andy. You can certainly exploit the CJ PV 11 true potential with better quality tubes.