Best Tube preamp under 2.5k Used

Sonic Frontiers Line series are excellent....you might one of them new in your price range.

Any Jadis preamp will be better than most any other brand at any price level. If the music is the most important thing to you, pick one up. There are several on Audiogon right now in your price range.

Sonic Frontier line 3 Preamp vs BAT5i which is best

For $2300 new the Melos SHA Maestro outperforms the BAT stuff and gives you the best sounding headphone amp in the world to boot.

CAT's are nice as a reference. Also Audible Illusions, and stuff from CJ, ARC, VTL. But if you really want to soften things up, get an old conradjohnson PV5. Not the most neutral, but wonderful for hard, electronic sounding stuff. About $500=$600 used.