Best tube preamplifier for under $15k

Wavac Pr-t1 
In my system clearly bested the Vitus Masterpiece, and fought a score draw with Boulder 2110.
The significantly more expensive Boulder was larger scale and has a wider envelope c/w subterranean, house-shaking bass, while the Wavac (with Amperex valves) was more musical, intimate and atmospheric....and is stunningly beautiful to behold

There has been a Shindo Vosne-Romanee lurking about Audiogon for a few weeks.

listed at 12k.  That is an amazing pre for well under your 15k budget.

like other posters I would also consider the 

Shindo Masseto can be found on here a few times a year usually at about half your budget.  The shindo line goes Auregies, Monbrison (regularly found on here for $4500-6k), Masseto, Vosne-Romanee, then a couple more above that but very rare.  The Masseto and the Vosnee are stunning.  especially if you can get a good deal on used.

models by VAC

KONDO Audionote (Japan, not the UK boys.... get the real deal if you can afford it.. sublime).  Every blue moon or so a Kondo will come on here that is in your price range.

Jadis (although I personally prefer Shindo)

Air Tight.. very very good and in your price range could likely get a great pre and a phone stage

There are others I would also look at.

Maybe a CJ GAT

Wavac

Few interesting German brands: Voxativ, Viking, others

Leben

Top end Luxman

As others have said; ARC.

There are more, but that would be a good start.

remember, you said best TUBE pre amps under 15k.

Any serious "best" list would have to include those brands and lines in tube pre amps. 

sailboat

Don Sachs all in pre.  
Or you can get an LTA integrated or separates and have plenty of cash to spare.  I have the Don Sachs paired with the LTA ZOTL 10 and its stunning.