Small or big tubes/valves for pre amp ?

Any 6F8G with 6SN7 adapters beats all 6SN7s IMO.
I personally like 12volt  tubes like 12AX7,12AU7,12AT7,12BH7...etc. better than 6SN7 . Like Lowrider said ,many of the  6SN7s that have good balance and Bass are microphonic.. like Ken Rad or Tung Sol.
The  vintage Sylvania 6SN7s can be  magical in all areas , except , they do NOT have good Bass. None of the Sylvania 6SN7s have good bass It is very weak . My amps which are the Granite Audio 864SR use 6SN7 in both V1 and V2.As long as I use something with good bass in V1              ( I actually use 6F8G) then using the Sylvania 6SN7-GT  in V2 works great  for me.

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 ;-)