^posted while I was typing the message below and I agree.
Note that we both suggest pretty much the same thing ..and I just assume people are listening while making any EQ or mixing choices.
Might first try simple high and low shelf filters, which work like traditional
bass and
treble controls.
I'm on a computer where I can't currently listen, but I'd try boosting somewhere below 200Hz and 10kHz and above to start. If you do that separately you can control how much in each region to taste. Cutting between there is the same in essence, and both may be used in combination, but does not give you separate control over how much bottom vs how much top on its own.
Scooping out the middle range makes more room for the X/Y pair to do its thing and may reduce some of the mid/upper-mid stridency. It also emphasizes the ranges in which the omnis tend to best contribute.
- Channels 5&6 summed to mono amplified to -8.0db
I'd not sum these. Not prior to mixing for sure, and I can't think of any particularly reason to do so afterward other than in the mix itself. Best to have the rear-facing channel(s) on a separate fader which can be muted easily and brought up last. Get everything else working as best it can, then sweeten with some of that. I generally keep the rear content level just below the point where it becomes noticeable, such that it is only really obvious in its absence.
Here's how I'd approach this mix:
The preparatory stage-
1) Start with the omnis alone. Hard pan them Left/Right. Balance the image by adjusting the level of each. EQ as necessary for them to sound good and well balanced. Add bass if lacking, add middle if it needs it even if you end up scooping the mids later. You can EQ each side somewhat differently if that helps balance things, just make sure it sounds natural with just the omnis alone.
2) Mute the omnis. Bring up the PAS X/Y pair alone. Hard pan them Left/Right. Balance the image between them by adjusting the level of each. EQ as necessary for them to sound good and well balanced, but don't worry about any lack of bass. This EQing may be more about fixing any overly aggressive mid or upper mid-range stuff that sticks out. If it helps you to get the EQ elsewhere adjusted correctly, bump up the bass some if it needs it, then remove the bass correction after you get the EQ across the rest of the range correct sounding. You can try it with that bass correction left in, but most of the bass should end up coming from the omnis and many times it works best if the directional channels are a bit lean at the bottom.
3) Mute the X/Y. Bring up the single center forward facing cardioid. Pan it to center. EQ as necessary for it to sound good. Don't worry about lack of bass. Same deal with addressing any aggressive frequency range that sticks out.
4) Mute the forward facing cardioid. Bring up the rear-facing cardioid. Pan it to center (1). EQ as necessary for it to sound good but don't worry about clarity of the music or vocals. It should sound natural, but like listening from far away. Often helps to cut mids and give this a bit of loudness type curve with more bass and highs. Mute the rear-facing channel.
The mixing stage (where the fun is)-
5) Unmute the omnis. Unmute the X/Y pair. Adjust levels of the pairs and play around with the balance between the two. Don't change the level relationship within each pair you established in the first stage, only between the two pairs. There may be a couple different balance relationships which work well. Listen for different things in deciding which works best- overall frequency balance, image width, center solidarity, image depth, etc. Listen for a while to get your brain familiarized with what you have. It may be good just like it is.
6) Unmute the single forward-facing cardioid. Adjust level and listen for what it does. Is it making things better or worse? Now is the time to have some comparative fun. Mute the X/Y pair. Go back and forth between just omnis and mono center cardioid and just omnis and center X/Y pair. Interesting. Maybe tweak some EQ on the mono center cardioid and see if you can improve just omnis + mono center cardioid. OK enough comparative fun, lets get back to the mix. As a monophonic channel the addition of this to the mix will solidify the center if that is needed, but compare that to keeping it muted and adjusting panning of the X/Y pair, bringing in each side from the previously hard-panned L/R positions (2). It might be better to keep the X/Y pair hard-panned and use some of the mono center cardioid to bolster the center image, maybe not. It might help to pan the mono center cardioid to one side to correct the overall balance with the other pairs unmuted. Maybe you don't end up using any of the mono center cardioid. Determine what works best.
(7) Unmute the rear-facing channel and bring it up. Listen for what it does. Listen for what goes wrong when its level is overly high. Bring it back down and determine if it makes things better when there is some but not too much of it. Find the level where it is no longer audible. Mute and unmute to see if it still makes a difference. This part will be strongly affected by the what is going on in the recording at the point in which you are listening. You are likely to want more of it during sparse low level music and between songs, and less when the music gets loud, or when there are distractions back there. (Aside- keeping as much direct sound arriving from the front out of the rear facing channel(s) as possible by way of your microphone arrangement reduces the need to ride this level as the program changes and makes it easier to decide on an appropriate level for it).
8 ) Save. Play around and revisit things and see if you can make it better. If you think you did make it better, come back later and compare with what you saved before you started second guessing.
Extra credit (a couple more advanced things you can try that often work well)-
9) See footnote (1) above. Adjust EQ on the rear-facing channel to as before as necessary, but instead of leaving it panned to center, duplicate it to an adjacent channel (or make it a stereo channel or whatever). Hard-pan these two channels Left/Right. Invert polarity on one of the two. Try this in place of a center panned monophonic rear facing channel. If it works you get more ambient width, a less cluttered center for the stuff from the front, and may be able to use more of that channel in the mix than you otherwise could. If it doesn't work, you can just go back to the monophonic rear channel panned to center. This technique or variations on it can also be useful when there is only a single monophonic center forward facing channel instead of a coincident pair.
10) See footnote (2) above. You can actually play with "over-panning" the X/Y pair "superwide" (maybe to make room for the monophonic center cardioid, or maybe just on its own). Doing so is essentially a Mid/Side ratio readjustment. So is simply panning both sides equally toward center which decreases width and increases "Mid", but this allows you to go wider than hard-panned. You can use a Mid/Side tool to do that and increase the level of Side, or your editor may change the standard pan control to a stereo-width control when the channel is a stereo channel rather than a mono channel. A stereo width control often has mono as one extreme position, normal hard panned L/R in its center position, and "over-width, super stereo" as the opposite extreme position from mono. Regardless of how its done, that kind of Mid/Side stereo-width adjustment to the X/Y pair is very useful for dialing in the perfect image blend between the omnis and the center. Its a very useful technique for any coincident pair, particularly well suited for optimizing a PAS X/Y center pair which might otherwise be a bit narrow on its own due to a narrow PAS mic angle.