I'm having great luck with my Juicy Music Peach II with a Cherry Plus as well. Just be sure you get the impedance matching correct, otherwise you may limit bandwidth. With your NuForces, the input impedance is around 22K I believe.
The Cherry line has 10K input impedance, which is why I went for a tube pre with a switchable low output impedance (90 ohms), which helps it play well with many different amps with even low input impedances.
I've heard the X250, neutral but a touch on the warm side. I can see how a warm, tubey preamp with the Pass might sound too warm. May be down to the pre, actually, as what I've heard from the Pass amps provides your desired mid/high warmth. The NuForce 9s that I tried in my system was on the cool/thin side, especially in the highs and lower mids, which would likely be accentuated with a "light" sounding tube pre. This was my experience with the "light" sounding Dodd battery pre and the NuForces.
Disagree strongly that Class D amps can't be warm. Have heard a Tripath based amp sound overly warm in fact.