On brass, I would personally skip the dynamics and go with a ribbon. BUT... that's not your question.
Trombone is not as bright as say trumpet. So you could get a little closer, but I'd probably start in the 3-feet away range, with the trombonist just off axis. You may get a little closer depending on the genre, keeping distance for a traditional trombone sound, and getting closer if you are wanting an in-your-face trombone with a little breakup distortion. The potential problem with dynamics in this situation is that you may lose the benefit of their proximity effect b/c you are so far out. I prefer a ribbon for this reason b/c it is naturally LPF'd in most circumstances (but not all ribbons are like this, some are very flat, for a ribbon).
Like your crazy uncle may have told you, and it applies here too, it's not what you have but how you use it.
9/1/2013, 8:50PM: A day and time that will live in avatar infamy...