Best small signal tube for a preamp?

I have a Zoe, and have acquired the following tubes thus far:

Sovtek 6H30pi
NOS Mullard 12AU7
NOS Siemens 12AT7 (early '60's)
NOS Sylvania 6BK7B

The Sovtek was warm, smooth and too weighty, and lacked some focus, detail, and dimensionality. I wouldn't say it's accurate.
The Mullard was a fantastic all-rounder. Rich & nice balance, but I was looking for more punch and midrange clarity. I'm now trying the Siemens. It's not yet broken in (20 hours or so). I'm very impressed so far. Very quiet, more 3d, solid bottom, and slightly more air, although w/ some grain and not as rich as Mullard's midrange (maybe after a few more days?). I've not yet tried the 6BK7B. About the others, I'm not sure if 6BZ7 is worth a try (the only one I can find is $8 a pair though), and I've read that the 12BH7 and 6922 are not low in microphonics and were not originally designed for pre-amp use... so I'm leaving them for later.
One odd thing I noticed when changing from 12AU7 to 12AT7 was that I had to turn the volume dial 40% higher with the 12AT7 to achieve similar volume levels. I was under the impression the 12AT7 tube has about 40% more gain than the 12AU7. So shouldn't it mean I should be turning the volume dial down instead? Anyways, I'll let you know when the Siemens has fully broken in.

Also don't forget the Zoe is a phase-inverting pre-amp, so you should reverse the polarity of your speaker cables (on one end) for optimum sound. Cheers.

Are you sure about the phase inversion? I did not know that. I will reverse leads and see what it sounds like. Keep me posted on your tube trials. I am in the process of getting a pair of NOS Amperex 6922's to try.

The manual doesn't say anything about phase inversion, and I don't have testing equipment to be 100% sure, but the Audiogon fellow I bought it from actually tests and writes technical papers for Soundstage magazine, so I'm taking his word for it. I'm very pleased with the Zoe, as it's right at home (center of it really) with the rest of my system, each costing more than Zoe.
I've read that NOS Amperex Bugle Boy 7AU7/7AT7 are some of the best. I'd love to also give Telefunkens a try. Can get used for about 1/2 price, which might be good as they usually have twice the life of others...

You may want to consider that many of these tubes have different load and gain levels and are not always interchangeable, even though they may look the same. If your preamp wiring cannot handle a load increase, you may incur some damage. For example, a 12AT7 and a 12AU7 look identical, however the 12AT7 has a gain of 60, while the 12AU7 has a gain of 19. If your preamp circuit was built for a 12AU7 and you retube it with a 12AT7, you are increasing the load gain to your preamp by over 300%.

Cyclonicman, I appreciate your comments but the Zoe is specifically made to operate with the range of tubes listed above. The preamp has a SATC (automatic tube adaptation) circuit and user switchable 6V/12V plate voltage, and the owner's manual specifically lists the tubes I listed above as all being compatable. One just has to remember to switch to the appropriate voltage, although yes the gains are different for the different tubes. Right now it sounds very good with some RCA 12AT7 black plates installed. I am still interested in trying a pair of premium NOS 6922 tubes, and currently looking into different sources.