Best tube preamplifier for under $15k

AR Ref 6.  I've not heard the upgraded SE version yet.  But the AR Ref 6 is a fantastic pre and you could pick one up used within your price point. 

I am with @roberjerman--I bought an SP-6 for less than $1000.00 recently and it is awesome.

Of course, the new Audio Research stuff may be better, but you will pay a lot more for them.  I would certainly audition one in my system.  They started out with the best of the best pre-amps back in the 1970's, and continue to produce better pre-amps than most.

Cheers!

Bought my ARC LS 15 back in 1995 and still using today and love it's musicality  Can't really remember,  but I think I paid less than $3,000

Jadis DEFY 7 used

Audio by Van Alstine Transcendence 10 SLR - $1299. Replace the stock EH 6922s with a pair of platinum grade National/Matsushita 7DJ8s for $120. Enjoy.

Oops, I read it as best tube preamplifier under $1.5k.