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.

That’s a generalization that large tubes sound better than small; it
depends on the implementation of the component. IMO, the 7308 which is a substitute for the 6922 sounds much better in terms of dynamics and realism. And most preamps use the 12AU7 due to the availability of NOS tubes, reliability, and good SQ in many different circuits.

The 6SN7, 6SN7GT, and its variants are very special; many types were ruggedized for military use in WWII, so they have excellent build quality. Their sonics vary greatly depending on the version and the manufacturer.
They have a large bottle and an octal base and may have the most pleasant and musical sonics of any preamp tube. They’re also excellent in the gain stage of amplifiers. Their greatest flaw, which is less common with small signal tubes, is that they can easily become microphonic. I admit it’s my favorite tube, but it does take more time and expense to find the right NOS due to the many different sonic signatures and also to test for low noise and microphonics.

I spoke to Don about purchasing his preamp, but it was a 3 month wait. There are many fans of his components online, not only because of the use of the 6SN7, but how it is implemented into his designs.

@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.

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.

@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?

@steakster brings up a notion that I didn't think about; auditioning some preamps. We've only been referring to 6SN7's and the Sachs preamp, but there's a world of tube preamps out there. Many fine 12AU7/ ECC82 based designs in which equivalents can be rolled-in. There are so many premium NOS tubes available that take a AU7 design to a higher level. I've used 5963, 5814A, 7316, CV4003, CV491.

With that said, I love the 6SN7. IME, no other tube sounds like it. I'm on my second preamp based around it and no matter what the design, it sounds heavenly. 

No, don't care for the GTA/GTB's.

I ran a single-gain-stage preamp with all brands of NOS 6SN7s and loved them (some more than others). But that design easily revealed microphonics. Using Herbies tube dampers sometimes helped.