It is a good idea to purchase a vintage tube pre-amp.

I previiously had an Audio Research LS26 which sounded superb.  Plus it is both single ended and ballanced.  Should be able to get a good used one for around $3000.  Can not do better for the money.

I have a Zesto Leto just like the one listed here on Audiogon. You can run it balanced or single ended. It's not the euphonic tube pre of vintage make but the company makes first rate electronics. The output impedance stays low so a long run to your amp is not a problem. It's on here for $3500 but you may be able to bargain. I have high efficiency speakers and you can turn it all the way up, dead quiet.
I paid a lot more for mine and would buy it again. 

With your budget you can pick at the top of the heap if you are willing to go a little older.  ARC SP-10 or SP-11.  While they are harder to find, there are even CAT SL-1s available in your price range.  I bought a used CAT SL-1 over 25 years ago and have never been tempted to upgrade it.  In all those years I've only had to replace one cap in the power supply.  Ken builds them to last.

The Harmon Kardon Citation 1 preamp is excellent when properly restored. To find one like that would probably be more expensive than the preamps listed, but it can keep up with them easily. A very competent circuit!

The Dyna PAS-3 is not a particularly great preamp, but it lends itself very well to update and modification! I've seen them be giant-killers on many occasions. We've done mods on them for decades and our UV-1 is heavily based on those mods. You can find used PAS-3s for $200-$400 depending on condition- and likely they will need refurbishment at that price.

Our own MP-1 has been in production since 1989. So I guess that can qualify as vintage; it features a direct-coupled output that allows it to be quite transparent; the real issue is whether you can find one.