Small or big tubes/valves for pre amp ?

Those Granite Audio are excellent amps.

When using NOS 6SN7's, only buy from a dealer who tests for noise and microphony.
Military grade 6SN7s are tested to a higher level, but even these can become microphonic. Some versions are built with extra supporting rods to help prevent vibration.
Variants include 6SN7WGT,
VT-231, and any tube with a JAN prefix. Tubes from the 1940s and 50's have the best sonics but are  more likely to be microphonic. That's why you need to buy from a reputable dealer. I buy from Andy at Vintage Tube Services; he specializes in early tubes and tests for the best quality and performance. I have several 6SN7GT's which are from WWII era and are excellent performers. Of course, these tubes are at premium prices.

I buy from Andy also.
Thanks for the compliment on the 864-SR
I also replaced the coupling caps on the 864-SR with Jupiter copper foil....
Not cheap , but Very Nice sounding. I have some JAN CHS-VT-231- 2 hole black plates that are my favorites , but still they lack bass
I like the JAN-CRP-6F8G-VT-99.

Just to clarify, I like a bit more detail than I hear at the moment with my setup,  but don’t want the details thrown in my face so to speak, just want to be better able to follow different instruments in a classical piece for example, not detail for detail sake ;-)

@gryphongryph , I listen to classical almost exclusively, I hear what you're saying. Good soundstaging and 3D imaging are important to me with orchestral music. And I want enough detail to hear the instruments in a realistic way. 

I spoke to Don Sachs and I've read many user reviews; his preamp would be perfect for you. He said it's very transparent with a black background. I didn't go with his preamp because an Audio Note came up for sale at the time.
  You should check out some of the threads related to the Sachs preamp.

Using 6SN7's in a cathode follower design, be prepared to do some tube rolling. There are many different combinations of tubes that will get you the desired sound. I love tube rolling, but it can get expensive with NOS.

@grey9hound , I'm using Tung-Sol JAN-6SN7GT, black oval plates/ black glass (1940s). It took Andy a while to find these tubes. He's such a great dealer and asset; I put in an order for the T-S's and he called me when he procured them. They're one of the "holy grail" tubes.
   Before that, I was using KEN RAD VT-231 1940's in my preamp (also purchased from Andy). All his tubes are low-noise and I've never had any microphonics (knock on wood).

You're using Raytheon with an adaptor? What's the sonic signature of these tubes?