kuba- Yes, you can easily adjust the overall tonal balance via EQ, but that's not the only way. You can also do so via the level balance between the omnis and the center mics. So for instance, with a quick and dirty mix straight out of the DR-680 recorder using it's built-in monitor mixer that provides no EQ ability, I can dial up the omnis a bit more and the center mic(s) down a bit to tilt the tonal balance towards more bass and less treble, or vice versa. Of course it helps if I'm actually listening while making those decisions. It happened to work out pretty well in the Youtube samples above just by knowing what monitor mix balances I've used in the past, even though I wasn't actually listening when I dialing it in for that patch-out.
This is where the flexibility in mixing OMT begins to come into play - and what really makes it advantageous in shaping whatever you come home with into something which works optimally for the listener.
Using EQ you can adjust overall tonal balance in a few ways- EQing each element separately before combining the channels and/or EQ'ing the resulting 2 channel mix after combining things. So for example you can EQ the omnis primarily for what they are contributing to the bottom end, while EQ'ing the center mic or pair more for what they are doing in the midrange and top end. And after combining the channels you can EQ the full 2-channel mix. Typically I do both, but with different things in mind..
Actually, I sort of consider three separate corrective 'steps' or stages in putting together the finished product, and EQ is an important element in each of those steps, second in importance only to general signal integrity and level. Looks like it's time to start outlining my process for mixing OMT. I'll offer you a basic outline of what I do before blathering more philosophy about mixing and this approach in particular.
Basic OMT mixing in 3-steps-
1) First I'll do base-line corrective stuff to each channel. Mentally asking myself things like- "Does this single channel have any problems?"; "Do the two omni channels sound similar in tonal balance and have a good level balance between them?" If not I'll compare just those channels and adjust one or the other as necessary to get them both sounding about the same. That imbalance could due to some particularity about where I was setup in the venue, the PA, my recording trims being off, a misbehaving microphone or whatever. In any case, I make sure each channel is okay on its own and more or less matches its pair in tonality if it has one. Then I'll make sure they work together well as a pair, balancing the level between the two so that the stereo image is centered when listing just to that pair on it's own. When I'm done with this step I have a good-sounding, well-balanced omni pair (channels 1 & 2), a good-sounding center channel (ch 3), a good sounding rear-facing channel (ch 4), and a good sounding pair of Left/Right supercardioid pair (ch's 5 & 6). If using a center forward-facing coincident-pair instead of front/rear-facing mics, channels 2 & 3 are treated as a pair instead of independent channels. Lock the individual channels of each of these pairs together so that you can adjust the level of the pair as a whole without upsetting its balance in the next step where you'll be balancing each pair against the other.
2) Then I'll do the mixing/combining part- setting up the mix balance between these elements. This is where it gets fun and is probably the most subjective part of putting everything together.
I usually start with the omni pair and build from that. We already have it sounding good on its own so just pull up the level on that pair. Hard-pan those omnis hard-Left and hard-Right. Don't worry about any "hole in the middle" of the stereo image at this point if there is one (probably not unless they were quite widely spaced). Next bring up your single center channel panned to center. You've already gotten that sounding decent tonally, so just listen to what it does as you raise the it's level and it combines with the omnis. Play around with it. I find there are often three ways to work it, it depends on the recording and type of music, and there is no wrong answer- either the center level is about the same as the omnis acting as an "equal player", the center is lower in level and basically "fixes whatever is lacking with the omnis", or the center level is higher than the omnis and sort of dominates the mix. I'll come back to this later as there are many different things going with this center/sides balance thing besides getting a solid stereo image, and the best choice will be one which finds an optimal balance between all those things. For now, just listen for a solid soundstage between Left to Right which a reinforces the center content nicely, such that everything sounds balanced and you really miss it when you mute the center channel. Don't worry about tonal concerns, how well you can understand the vocals, how distant it sounds or things like that at this point.
If you've recorded a center coincident-pair instead of a single center channel, pan both channels of the pair panned to center. That effectively makes it a single monophonic center channel. Balance it's level with the omnis as described above first, then play with panning the center Left and Right channels outwards by the same amount (symmetrically). Try it fully hard-panned to either side as well as in all positions in-between. The nice thing about having a coincident center pair is that you can pan this pair however you want without creating phase-conflicts. Listen for a good, even image-blend between the center and sides. You might like it best with no center spread at all (both mics of the center pair panned center), with fully hard-panned to either side, or somewhere inbetween. Again there is no rule here other than getting it sounding good. In general you are likely to end up with a center pair panned less widely than you normally would without the omnis in the mix. Sometimes I'll pan them just a minimal bit to either side, but that slight panning makes a big difference in getting a smooth blend across the full soundstage and keeping the center from sounding separate and overly point-like. If you mute the omni channels you might be surprised to find how narrow your choice sounds on its own. That's an indication of how mic configurations need to change when used in combination with one another (taken care of by the OMT mic setup) and is helpful to hear to understand how each of the parts combine to make a sum greater than the parts.
Take your time here, work up a few different balances between center/sides (and center spread if you have a pair there) and listen to them for a while to see which one works best and sticks with you. Often it may become apparent that what was sounding really good at first doesn't hold up under longer listening or only works for that particular section or song. Try to find whatever balance works well for the entire concert and holds your interest without anything about it bothering you after a listening for a while.
If you have a rear-facing microphone, bring that up last panned to center. The optimal level on the rear-facing channel will vary a lot depending on the situation, the room, the audience, etc. You'll probably want to use more of it when listening to the sections between songs and during quieter numbers when the audience is quiet, and less of that during louder numbers, or when the audience is talking back there.
3) Once you have a mix you like, you can fine-tune the 2-channel mix bus or the resulting two channel mix output if you choose to edit that separately. We are now to the point were we do the same things we would to any normal two-channel recording- EQ the whole thing (more subtle overall-tonal tweaks at this point), normalize, track, fade, whatever.
To me it helps to think of those 3 steps as follows:
The first step mostly about "fixing problems" and getting to a good level playing field starting point.
The second step is the "creative mixing" part where most of the gross (and subtle) decisions which have the greatest creative input on the the result are made.
The final step is the "mastering" part. Putting the final polish on the thing and otherwise prep'ing it for release.