Tube Pre/ Class D Amp - Who has done it?

I quite like my PS Audio M1200 mono amps and preferred them to my PSA BHK250. It synergistically plays well with my DeHavilland UltraVerve and ModWright LS36.5 preamps. It also sounded great with the departed PS Audio BHK preamp. Sounds excellent with the solid state Luxman M700 as well. The ability to  change the M1200's input tube helps fine tune the pairing with each preamp. Plus, as I get older, I really appreciate only having to lift amps weighing 30 lbs. instead of 75 lbs.!

Hot-rodded VTA SP-14, with Bel Canto REF600M class D monoblocks into Proac D30rs

What a great question.   Sometimes this works, it's not always so easy.  I had a rogue tube pre going into 2 mono blocks from Auralic and it was very good, but lacked a bit of detail.  Soundstage was huge and imaging very good as well.  When I swapped in a solid state amp from ML when they were called Proceed, the sound was surgical in its precision, but lost some warmth and bloom.  

definitely worth experimenting, but IMO you can get superb sound with a SS pre and do not necessarily need a tube pre. 

I had an AR 150M (class D) with an AR LS26 (tube) running electrostatics and it sounded sublime.

I had a Modwright tube preamp with a Bel Canto class D amp----lots of power but I kept getting some noise out of the preamp. Traded them in for a Luxman integrated.