Best tube preamplifier for under $15k

There is no best anything.

@kiethr

The VAC Renaissance V with phono is my current preamp.  Simple remote with mute and volume provides me the convenience I desire, but still have to manually change source.  The transformer output is able to match to any power amp and currently drives my Ayre VX5/20.  Another solid feature is the phono section that VAC is know for and it performs well with my Madake, Purpleheart, or Windfeld Ti cartridge.  You should seek out a demo or at the least call VAC and speak with Kevin or Brent about your system goals.  I have owned 3 VAC preamps over the last 15 years.  Factory support is excellent no matter what level of their equipment you own.   Sorry to sound like a crazy fan but I guess I am...
Rick

@jahatl513

I am glad you are pleased with your LTA microZOTL.  I think it is a fine preamp, but in my situation it was not the best fit.  Different strokes for different folks.  And our natural variability in preference and hearing make for an even richer audio community in my opinion.  
Cheers

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