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