Best value in pre-owned preamps right now?

Mmm depends on your budget as this is all relative. My picks would be the Audio Research Reference 5 if you want a tubed preamp & Vitus RD-100 if you also want a good dac. They can be found had $6k 2nd hand. The Ref 5 can also be upgraded to SE spec later & tube-rolled if you wanted to take it further. But if you're after a pure ss linestage, the Vitus RL-101 is amazing for the price & is based on filter down technology from the Signature and Masterpiece series.

.
My vote goes to the Audio Research Reference 3.
Right at $5k and sometimes a little less, it is a stunner.

The new Reference 6 is $14k.
.

Another vote for the Modwright. Best pre Ive owned.

Spectral DMC-10. Particularly if you need a phono preamp. Forget the DMC-6. All Class A discrete design with a real tape monitor loop. Blindingly fast. No remote. Manual volume control an input selector only.

You can probably find one for under $1500.

Cary SLP05.  $4k and it is a good one.