Small or big tubes/valves for pre amp ?

6SN7's have such a wonderful timbre and overtones. Since you are familiar with Mullard but like detail, Brimar is the better tube. It's a UK tube similar to Mullard, but with more detail and extension.

The Sachs preamp uses a cathode follower so the sound would be very clean and revealing. With my Rogue I would use a pair of matched tubes in the gain stage with a different brand of tubes as drivers. For example, Amperex in front and Mullard as drivers to add some warmth.

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.