Small or big tubes/valves for pre amp ?

@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?

@gryphongryph I’m also a fan of Van Morrison. When I swap tubes, I use cut 2, Philosophers’ Stone from Back on Top, as one of my references. On this cut, he’s very close mic’d.

I have a tube preamp (6922’s) with a tube DAC (12AU7’s) along with a Class A solid state amp. I’ve probably swapped about 20 different brands of NOS tubes. Depending on the tube (and cabling), his voice on this cut can be hot or recessed or just right.   But, more than naturally reproducing a male or female vocalist, how is the PRaT?
Does a snare drum sound crisp?  It's a challenge to balance everything out.

I used to enjoy NOS Mullards 12AU7’s. Then, I went on a binge to seriously clean up the power to my rig. After doing so, the Mullards sounded overly warm with low resolution. It turns out they were masking a problem that I had with dirty power.

I totally agree with lowrider57.

That’s a generalization that large tubes sound better than small; it depends on the implementation of the component.

It would be ideal if you could audition a preamp in your system before purchase. Are there any audio clubs in your vicinity?

@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. 

I think this is very close to my observations here
https://www.audioasylum.com/messages/tubes/221750/6f8g-vt99-observations

The 6SN7 Tung Sol and the Ken Rads , that Andy had, at least one of the tube in the pair , was somewhat .
microphonic in my system. If you can find those that are not microphonic, the Ken Rads and the Tung Sol are the 2 brands that actually have Bass that I can live with , most all of the other 6SN7s do not ...IMO
This is what they look like .. this is not my  amp . Scroll down a little bit 
https://www.head-fi.org/threads/wa22-lets-roll-some-tubes-done-mod-burnning-in.451678/page-27