i don't know how much the tube preamp is going to add to the quality of the sound--at that point (after the POD processing) the dynamics are pretty much set in stone so maybe it will act as a filter to change some of the tonal characteristics (might be good change, might be bad change.)  because the POD is a preamp itself you probably won't be adding significant gain to the signal.  if you really want tubes in the chain consider putting the money into a tube power amp.

You'd be better off with a tube power amp and skip the the tube preamp. I used to run my X3L stereo into a Carvin TS100 tube power amp and 2 4X12 cabs. I got great results, but it was too much to lug around.

I have an ART Project Series tube preamp that I occasionally use with my PODxt.  My method of use is to place it before the POD. Signal chain looks like this: Guitar - - - > ART - - - > PODxt - - - > T21 Power Engine.  Mainly, I was looking to add some warmth and overall "roundness" to my POD tone, and in this capacity the set-up seemed to work pretty well.  I'll also add that the T21 Power Engine is a 60W solid-state powered extension cabinet, meaning that I'm not sure how the pre-amp thing will play out with a larger, rack-mounted power amp or running out to a board.  I have, however, recorded direct using this set-up and I though it sounded good.

As noted in pbear5's post, the PODxt is already modeling all of the primary dynamics of the amp, i.e. the pre-amp / power sections, the speaker, and the cabinet. Along with that, its also modeling the room (A.I.R.) and microphone.  As one might expect, these things will all require a bit of tweaking to sound their best in conjunction with whatever you decide to run in front of the POD.

The amp models that really seemed to benefit most from my own set-up were the clean models - esp. the Fender stuff.  I also liked the sound of single-coils better than HB's with the pre-amp engaged.  They seemed to sound fuller and more articulate.  As for tones, I was able to get a nice, slightly driven break-up by pulling the drive down on the POD and pushing the signal a bit with the tube pre-amp's gain.  Using this approach, I dialed in some pretty cool Stones & Black Crowes stuff.  I wasn't as fond of the high gain models with the pre-amp in the signal chain - though admittedly, I'm not a high-gain player.

Everyone's ears hear different things, so results, of course, may vary.  With a bit of experimentation and logical application, a tube pre-amp might be just the ticket to add some warm, "tube-ish" subtleties to your tone.