I agree with Jazzcourier about the Bendix 6900, but the only pair I could find was over $600, so I passed. They are mega-rare and mega-expensive.
I did a MASSIVE amount of research before, during and after I bought my 9.0 Sig and found alot of info about tubes. Dan Wright told me that I hadn't really heard the preamp until I heard it with the Mullard 5AR4, so that recommendation came straight from the designer. The Mullard has a deeper, richer tonality than the Sovtek and just sounds "right" to me.
The 5687 tube is pretty linear, (just like the 6H30 tube), which means it has more solid state characteristics to it. The Amperex 7119 was smoother, more open and threw a wider and deeper soundstage. It sounded more "tubey" to me, which is what I was looking for.
My advice to to try some different tubes since they are really not that expensive, (except for the 6900). No one can tell you what YOU like. Everyone's system and room are different.
I would do some more research and try to narrow down what type of sound you are looking for. There is a bunch of information on this preamp out there.
The tubes that will work are the 5687, 7044 and the 7119.