Tube pre-amps: what defines "better"

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.

Jafox, Gs5556 said, "Generally speaking," implying his awareness of anomalies such as the one you identify. There always have been instances where companies charge more because their brand name recognition allows them to, notwithstanding better quality available from other (perhaps less commercially recognized) manufacturers at comparable (or lower) price points, viz., your example of AR vs. BAT (with which I concur).

To get back to Jpaik's question, besides power supply, other issues significantly affecting cost (other than fit-and-finish and overall build quality) are the nature and quality of the volume control (beyond tube choice, volume control is another way in which many preamps add their own personality, color or even noise) and the quality of the internal wiring (point to point vs. circuit boards, e.g.).

I also add my voice in support of those championing the AI gear. I have owned both L1 and M3 in my time, and both provided outstanding sound and quality for the price, as well as being quite customizable in terms of sonic character with tube rolling.

Like many things in life, the better ones devote more attention to detail - both in their manufacture/design and their sound. I have upgraded my tube preamps from a Quad 22 (say 150 USD now) to a Verdier Control B with valve PSU (2300 GBP 4000 USD) to my current Tron Meteor (6000 GBP 11000 USD - replaced by the New TRON Syren - www.tron-electric.com). The mass of the preamps has increased considerably to reflect the better PSUs. However, more is not always better. It is important to get the details in the design correct - good quality power supplies, top grade or bespoke components. short signal paths which are correctly aligned to minimise noise etc. Inevitably this costs money and time. My Tron Meteor preamp took over 70 hours to build and weighs several times that of the Verdier let alone the Quad.
So what do I hear - very little! It's as if all the grain in the sound has been removed. The sound is transparent without noise. Soundstaging is natural and the Tron makes everything sound more like real music. That's what it's all about.

the rest of my system is
Platine Verdier Schroeder 2 Allaerts MC1B, Quad IIs (GT Audio rebuilt), Avantgarde Duos with PHY-HP cabling

I've been using Conrad Johnson tube preamps since the mid-l980s -- first the Premier 3 and now the Premier 14 -- and when I upgrade it will be to another CJ. Good luck, Dave