Tube pre-amps: what defines "better"

Musicslug is right about different tubes having their own character. I'm pretty familiar with two types, the 6SN7 and 6922 families which my preamp and DAC use respectively.

In general, the 6SN7 will have a fatter midrange and upper bass which yields a lightly more organic sound than the 6922, 6DJ8, 7308 family which on the whole is more neutral sounding. I like both!

When considering tube preamps, pay attention to the tubes being used in its design.

Generally speaking, the more a tubed preamp costs, the better the power supply. More often than not, the preamps' performance will depend more on the power supply and voltage regulation than the types of tubes it uses. Regulating voltage and providing clean power is costly in terms of components used and construction. This pretty much applies to all active audio components.

Real world example: a friend purchased a used Melos tubed preamp for around 700. I purchased a used Audible Illusions Modulus 3A for 1200. We plugged both of them into a aystem with very good speakers and a custom interstage coupled 300b SET (best we've heard and that includes Kondo). Our findings when we took the Melos out and inserted the Modulus 3A; the midrange suddenly had detail and transparancey again. The bass was tighter. The Melos has already been inspected and pronounced in fine working order; the Modulus has its stock tubes from 4 years ago and they s/b changed out.

The only tubed preamps I was considerign in the used market when I finally bought the Audible Illusions Modulus 3A (mostly because it was available and the other two are next to impossible to find used):

Audible Illusions 3A
Audio Research SP9
VTL Ultimate (first on the list, hardest to find)
CAT SL1 Signature or one of the other SL1 models in phono. This one is significantly more expensive than the others.

The Audio Research preamps (SP6, 8, 9) are excellent units but, at their age, certainly should be put on a bench and gone through as parts probably require replacing and/or upgrading. I'd budget an additional 500 max for this task once the unit is acquired.

The Modulus 3A is a great unit but uses up tubes a bit quicker than others. A good set of upgraded NOS Siemens EC88 (6922) tubes is about 100/ea and you need 4 I think; have not installed mine yet.

My finding was that there are not that many options in the 1000 to 1500 range for a quality used high end tube preamp if one insists on a tubed phono section.

If you have patience I'd counsel wait for a VTL Ultimate.

Too me, the more expensive tube preamps, assuming they are of good quality, is the ability to play louder with less conjestion and/or decreased dynamics. As stated above, I think this has to do with the power supplies more than anything else.
The AI M3A preamp is awfully good for the money it goes for used. It still stands comparison to some of the best available. If you don't need a phono section, then the 2 tube L-1 is nothing but a bargain used. I have owned both of these for long periods. The L-1 has just a small amount of that tube thing going on where the M3A is very neutral for a tube product. Also, either of them change with the use of other brand tubes. You can taylor the sound somewhat.
I curently use an Aesthetix preamp that costs almost twice what the L1/M3a does. Yes, it is just a little cleaner through the mids and has firmer bass but it is in its ability to play louder where it stands out.
I have always used a tubed preamp with a solid state amp. It kinda of gives you the best of both worlds. The only solid state preamps I have heard that I consider decent is Ayre's K5xe and K1xe. Other than those, too many SS colorations imo. I have heard a couple of decent passives.

Gs5556: I would agree very much with the importance of power supply design in tube preamps. But unfortunately high cost does not always imply a beefy power supply. Take the covers off the ARC LS25 and the BAT31SE, very comparatively priced products, and you will see the enormous attention to the BAT's power supply compared to the ARC. The same is true for the BAT 51SE compared to the Ref2.