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.
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. 
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@elizabeth I hear what you are saying, and for some manufacturers this is fine, from Schiit (or is it iFi?) that have the sockets. But i think they might limit the quality the OP is after.

I’m not a purist, but I dont use the sockets that allow other tube types, b/c it is just another thing in the way of the audio chain. And the OP is looking for good tubes with good bang for buck. IOWs, I personally would not put a socket on a $75 NOS tube.

TBH, im not familiar with your CJ PV11, so i did a quick look up- this is from the 90s? and uses 3 different tube types? of course it is fine to have an older model, but that will tell us more about its general circuitry under the hood. Some amps can handle substitutions, some cant. Some pre amps now (LTA MicroZotl) have a choice of 6SN7 or 12AuX, b/c the circuit board and electronics inside support these different electrical platforms.

@rvpiano: i would like to make some recs on tubes, but it would help me/us to know:
1. Do you have a budget per tube or to replace all 6 right now? or replace just some? Whatever you do, swap out all the tubes of the SAME type. eg, dont replace one 12AU7, replace all of them at the same time. you probably guessed this, but some people dont.
2. I know the Nuforce class D amps you have, but what speakers do you have?
3. What particular aspects of sound quality do you want to change? more or less hi frequency? more or less mids? more ore less bass? more balance overall? a "sweeter" sound? a dryer sound? a colder sound? a bigger soundstage? a smaller stage? a more aggressive or laid back sound?  you get the picture....