I recorded a concert of a choir with a backing band and brass line.  The issue I am faced with is that, with the choir mics down, the band sounds great.  All of the instruments were either close miced, or direct, and the players are all for-hire professionals.  When the choir mics come up, that nice tight dry sound is mixed with the revereberant mud of the auditorium.  The instrumental bleed into the choir mics is nearly as loud as the choir itself, particularly the drums, which were not shielded or caged.  I have put a high pass on the choir mic group, which takes out some of the bass guitar and kick drum, but I'm at a loss for how to reduce the overall ambient reverb of the room without gutting the choral sound.
 
Anyone have any tricks (aside from what larger budget "live" recordings do, which is re-record the vocals in-studio) to cancel out the reverb?

Thats a hard one but you may want to try R Mix because it can do some amazing things. 

You could try a Gate, but removing the room ambiance and track bleed from an audio file cannot be done 100% and it will also harm the track (choir). This is why the room is so important, as the room has a big effect on how the instruemnts/vocals you are recording will sound.
 
CJ

Unveil, from Zynaptic, is amazing... not cheap, but it will remove more ambience, with fewer artifacts, than any tool I've tried.

Cool, guys.  Thanks!

• i use iztope rx2for sermons to remove some of the room from the lav. mic my pastor uses it can remove some of the room 

That's a tough scenario...
I don't think you'll find any processing that'll remove a significant amount of reverb/"noise" without seriously altering the sound of the choir.
I'd start by running that high-pass as high as absolutely possible on the choir.
That'll get rid of a lot of mud...
Otherwise, make extensive use of volume nodes to tuck away as much "noise" as possible.

Jim,
 
Check out Unveil.  It's pretty amazing.  I did not know ambience removal was such a big deal, but then I'm not a video/film producer :-) They seem to have to deal with this all the time due to on-set ambience issues, especially when ADR is not an option.  Unveil has some impressive demos out there, and specifically refers to removing "mud" from a recording.  A choir in a large hall is an incredibly complex and dense signal.  I can't wait to see what Unveil can do to clarify the recording. We normally bring in a group of vocalists to overdub the live performance in the studio, whereby we can reduce the live micing in the mix to only that amount that makes it what people expect from a "live" recording.  This time we just don't have the time or budget.

Unless you spend the money on the above tool suggestions and let us know how they work, you'll probably have to go the old fashioned route.
 
High pass until it starts biting into fundamentals, then back off.  Same w/ the gate.  Unless your choir is moaning at low levels, you should be able to clear a lot of the muck out this way.  If you want to do it proper, get an impulse of the room and add back in some reverb on your choir unless you like how it sounds almost nekid.  Otherwise try to match an algorithmic reverb.  R-Mix can also work wonders - some times.  And if the music/choir is right, you might be able to notch some frequencies out, tho I wouldn't count on it.  And don't forget vol envelope for those spots where the choir isn't sounding at all.  You will probably want to ramp the 'verb up and down, rather than cut it.  You are just going to have to throw the kitchen sink at it and see what sticks, if you get the mixed metaphor.
 
@

wst3
Unveil, from Zynaptic, is amazing... not cheap, but it will remove more ambiance, with fewer artifacts, than any tool I've tried.

The demo video is pretty amazing.  DO you think this tool could be helpful in filtering out niose from live recordings?.  I've had cases with motorcycles in the background, or thousands of cicadas.  It seems like tool can magically find the real program and separate it from the background.