Let me just give a general answer here, because many people might end up in this situation today or tomorrow.
You could do several things to get rid of this boomy sound. In my opinion, you are using too many microphones to do this recording with.
First, two items apart from microphone placement.
Getting rid of the boominess
If you record in this same room often, get your hands on a measurement microphone and record a sweep from a studio monitor in this room. That should give you an impression of the room's frequency response. Now, you can do a basic frequency analysis (most DAWs are capable of doing this) on the signal and design an inverse filter for it. Don't overdo this though, digital filtering to get rid of acoustic shortcomings is quite a nasty thing to do.
To get rid of the low frequencies in the hall acoustically, try putting Helmholtz absorbers near the corners of the room. Additionally, try to get rid of as many reflections as possible by hanging curtains on the walls. A carpet can do miracles as well. This is especially a good idea in square rooms, or rooms that have parallell walls.
Then, make sure your microphones don't pick up any sound of frequencies that can not be produced by your musicians. Most men can't reach frequencies below 100 Hz, so if your microphones come with a 120 Hz roll-off filter or cut-off filter, try applying that on your support microphones. Especially if you are using cardioid (or any other non-omni) microphone that is placed closely to the singers, do this to get rid of the microphone's Proximity effect. (Don't do this immediately on your main microphones, they shouldn't have this problem in the first place.)
On mic placement: I had very good results with multiple close XY pairs; you get as much directionality as you want (because you are allowed to pan XY pairs) without comb filter effects due to phasing problems, while you can still keep your balance. You didn't put any information on how the signal levels were in the mix of your support microphones. If you put them too high, the comb filters that you get by mixing two microphones together that are close, but not at the same place might well be part of the source of your problem.
I would start with the following setup:
For main microphones, I would prefer AB over XY. (If less than 25 singers or if the room is very reverberant, I would go for a hybrid system like ORTF though, I've had very good results with the Schoeps MK-21 semi-cardioid for this purpose, but any semi or cardioid should do fine. Don't use figure-8 or super/hypercardioids, this is generally a bad idea). Make sure you are aware of the microphone distance to get the correct SRA. (Stereo recording angle.) If you don't know how to do this, it basically comes down to "listen, and adjust". Basic rule: the closer the main microphones are to the choir, the smaller the distance should be. This may sound unlogical, but, well, unless I am writing another long story, you'll have to do with "It just works that way". Keep the microphone distance between 51 and 70 cm, but in this case, I would suggest to put the microphones closeer to 51 cm. Again: keep listening and adjusting where needed.
General rule about AB systems: never check your stereo image quality using headphones. While you can get a good impression when using an XY, when using AB, the stereo image gets widened by 80% when listening through headphones, so you really can't here when you are using the right microphone distance. Always use a stereo setup of monitors. Don't have monitors? As a general rule: 51 cm distance records 180˚, 70cm records 90˚. Roughly estimate the OVA (operational viewing angle) of your choir from the spot where you put the AB, and then make an educated guess for a microphone distance.
Make sure your main microphones are of the best type you have available, and using the best preamps you have available.
Then, I would put 2 XY pairs (3 if the choir has > 80 people), one on the left side, one on the right side, both pairs being at least 2 meters distanced from each other, to avoid - yes, again - the comb filter effect. Start panning the XY on the left to L-C and the right XY to C-R.
Place these microphones facing the singers rather than above them.
I would get rid of the microphones behind the choir. Unless you have a good philosophy for putting them there, I think they really are not recording sound that is of great contribution in your mix.
You don't necessarily have the best result with XYs that have angles of 90˚. If you place them really close, you might get good results with much lower, or much higher angles. Play around with this a little.
If you want to pick up the accoustics from this room, you would be better of getting two omnis or cardioids in the corners behind the main microphone rather than placing them behind the choir.
Always start with listening only to your main microphones. If you don't like what you hear in terms of acoustical balance (between direct sound and reflective or reverberant sound) or in terms of stereo width or acoustical disturbing signals, fix it in the placement. You don't want to fix these issues with your support microphones, that's not what they are there for. Also, don't apply EQ to your main microphones, (at least, not initially) just put some real effort in getting the best available sound using these microphones.
Then mix the support microphones in. Try to use the support microphones only to fix balancing issues and to improve the overall intelligibility. If you end up with something that you don't like, start with shutting these mics down, and find out where it went wrong. Just a bit like software debugging.
So, a few comments on your question:
- I think you are using far too many mics for this purpose. (Personal opinion, people may well disagree with me, but I strongly believe the additional value of many microphones is very limited.)
- The overhead from behind mics are expected not to be of great contribution as support microphones.
- Support microphones might be too close together, so when mixed into the same channel (either L and/or R) they can introduce comb filter effects, which could be part of the cause of the sound issues.
- Main microphone XY wouldn't be my choice either on a choir, while it records stereo very precise and analytically, it doesn't give you any depth information (because there is no time-based stereo.)