Looking for warm tube preamp (should pair well with Pass power amp), budget $6000

BAT, Atma-Sphere or Conrad-Johnson.

Aric Audio as @cmach pointed out. I run the Motherlode II with First Watt amps.  I find it to be an excellent pairing.  

When you say "warm preamp" I don't follow.

If you said, "warm tubes for preamp" I would follow better. But maybe there is such a thing as a "warm preamp" above and beyond the tubes put into it.

If that is true, I'd appreciate folks teaching me that.

I owned a Audio Research 5SE for many years, with a Pass amp. The best thing I did was to get rid of the Pass amp and replace it with an ARC amp. Honestly, most likely you are thinking of replacing the wrong component. Replace your amp with a used Ref75… or better. this will have the effect you are looking for. Audio Research Reference preamps above 3 are just plain world class… their amps are just amazing… I now have a ARC Ref 6SE. And a Ref 160s amp… although I have been using ARC Ref 160m monoblocks for the last couple months. These are an incredible combo.