Small or big tubes/valves for pre amp ?

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?

@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.