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.
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!
I have always gotten solid advice and feedback from Brent Jesse, Andy at Vintage Tube Services, and Upscale Audio. I’ve purchased from all three over the past 20 years, and have not gotten a bad tube from any of the three. 

I would start by replacing the two 12AU7s in the line stage with a very high quality matched pair, as all sources will run through that preamp stage. Replace each additional tube type in phono section in stages to evaluate what each is doing to your sound for your future reference. You may notice some tube positions make larger differences than others.  Have fun!
I've had three different CJ tube preamps over the years.  I found that European as well as some Japanese NOS tubes made a significant difference/improvement.  Be careful to get low noise tubes, particularly in the phono section.
Having tube rolled integrated amps, phono stages, and amplifiers, I can say that tubes can make all the difference in terms of gain, clarity and other aspects. The tubes in your CJ pre will make the most difference. I replaced the stock 12ax7 tubes in my pre with GE 5751 tubes and it made an incredible difference in terms of clarity, gain, presence, and soundstage. Those tubes were $60 each, but it was an immeasurable investment.

But I also did my research in terms of what tubes were the most complementary to my unit beforehand.