Small or big tubes/valves for pre amp ?

I have had some correspondence with Don, he seam like genuine nice person and very helpful, but of course he will not talk down his own product, and nor should he.
As I know the the flavor of many nos 6922 and they can be very different, I have no knowledge about the 6SN7 family of tubes, have heard also that they are a bit more fragile than the smaller tubes and do not last as long.
I like the Mullard sound, but with detail, am allergic to thin/shrill sound.
Tone, decay, timbre and natural voices is something I listen for in the music, I find that many amps and speakers today can replay a woman voice perfectly, sometimes spellbindingly good, but male voices not so good, often sounding neck up only, no chest!

I have my dream speakers now, and with my solid state amp I have problem with Van Morrison for example, sounding too thin, but with my hybrid amp he sounds fantastic!
Going to a tube pre into a solid state amp, a Belles or Van Alstine I am hoping to get more detail and maybe a more euphoric sound, also my hybrid amp makes me listen to music for hours at a time, my solid state amp, not so much! 

Go for it.
Use Psvane Globe 6SN7 from Grant Fidelity. You might want to ensure the pre can take their size. They are amazingly detailed.

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.