I've used a tube preamp with a couple different amps -- an older Adcom 535L and a new Van Alstine SET 400. Both give good grip on the bass and quick timing, but the midrange and upper range were not smooth enough for my ears. I'm running KT 150 tubes on my monos now and it's all working well.
However, if a Pass or Ayre or Hegel amp come my way, I'm curious to hear those mids and treble ranges smooth out considerably.