Ayre V-5xe best tube pre-amp to match with?

Whatever choice you make get a truly differential balanced preamp. The Ayre gear is meant to be run with a balanced signal and although it sounds okay single-ended balanced is the way to go. The mere presence of XLR outputs on the preamp does not necessarily imply a balanced signal. Atma-Sphere, Audio Research and BAT are balanced and I believe Aesthetix is too. CAT, ModWright, Herron and (most- perhaps all) VTL are single-ended.
Ultimately there are better preamps than the K-5xe or K-3x but within their price range I haven't heard a preamp that is absolutely better. The affordable tube preamps are more different than better in any absolute sense.

Frankpiet - I had trouble with a DeHavilland Ultra Verve when matching with Ayre. This is in NO way a knock on the DeHavilland as it worked fine with another SS amp, and at it's price I think it's one of the best buys out there.. but for some reason it and the Ayre wern't compatible.(Hum) The deHavilland is single ended.

Just heard Hovland HP-100, Nagra PL-L and Aesthetix Calypso (have heard all before but never against each other in one system except of the latter one). Hovland (less dynamic and less resolution and tightness than the others but a midrange to die for). Nagra to analytical but extremely tight bass, great micro / macro dynamics and air around Instruments, tons of details - but surprisingly quite uninvolving. Calypso despite the glowing reviews quite flat and noisy (probably stock tubes were not burned in, but great ergonomics and nice to look at). I´m very keen to listen to CAT SL-1, Ayre K-1x and BAT VK-31 SE.

Check also Cary slp98, Audionet (the big one).

I found the 51SE to be quite a big step up from the 31SE. YMMV.