My thoughts-
The clarity and stereo image in the OMT5 mix really grabs me. I have a soft spot for crossed hypercardioids, as they often get a lot of things right to my way of thinking, sounding really good to me in a number of important ways, and I think this particular set up choice of angle and pattern is well chosen. In addition to the stereo imaging aspects, there is a very nice clarity, heard especially in the vocals and percussion transients, and the stereo ambient pickup to the rear is nicely balanced against the direct sound arriving from the front.
Part of that is related to the high-quality 426>V3>(SPDIF maybe?)>DR680 signal chain, but a lot of it reflects aspects of the microphone configuration itself. I've posted previously about how I consider crossed hypers as being something akin to improved Blumlein crossed 8's for real-world taper situations, in that it provides similar imaging and ambient qualities, but with an increased forward-sensitivity bias which makes the perfect placement in a perfect room which Blumlein requires far less critical.
Similar to Blumlein, crossed hypercardioids is sort of like using four center microphones (a forward facing pair + a rear facing pair) instead of two, but with the mix of the four being fixed. The crossed rear lobes of the hypercard pattern serve a similar role as dedicated rear-facing microphone channel(s). The difference is that the front/back reverberant balance is fixed, or rather adjusted beforehand by choice of the pickup pattern and the angle between microphones, rather than via mix level of a separate rear-facing microphone (or rear-facing pair). For those unfamiliar with the microphone, the single-point-stereo AKG426 provides a choice between 9 (IIRC) electronically switchable pickup patterns ranging from omni to figure-8, which may be set separately for either channel, as well as variable angle between the microphone capsules.
In an OMT setup, the narrow included angle of the central X/Y pair works well for a couple reasons: It points this coincident center pair of microphones so that they are more-or-less on-axis with the primary sound radiators (the PA stacks), providing good direct sound clarity; and it solidifies the center without concerns about loss of spaciousness or stereo width carried by the omnis. Addition of the forward-facing AKGck61 further increases forward bias and center solidity. And in a practical sense it provides an additional degree of freedom in the mix.
Kyle, I'm most curious about your thoughts while mixing this concerning the included angle of the 426 in combination with the center ck61:
Do you think you would have used as much ck61 in the mix if the 426 had been set to a slightly more forward directional supercardioid pattern?
Similarity, do you think you would have used as much ck61 in the mix if the 426 had been set to a slightly narrower inclusive angle?
^Those questions hint at the possibility of OMT4 incorporating just the 426 + omnis. Curious if you speculate that if the most-appropriate pattern/angle combination for the 426 could be determined beforehand (a big if), no forward facing ck61 would be needed.
Alternately, going the opposite way:
Assuming the center ck61 is not "optional" but included as a necessary addition in the microphone setup and mix (with or without a rear facing mic), I wonder about a wider inclusive angle with the 426. I could see an "over wide" angle of greater than 90 degrees working well as long as the center ck61 is there to hold the center in place. That would have the front/back facing center pair playing a bigger role as necessary additions to the directional stereo imaging and stereo ambience provided by the 426. The 426 would no longer be PAS or near PAS, which may be less appropriate in term of direct clarity from a relatively distant recording position in a reverberant room, but it would reduce interaction between 3 the similarly pointed, close-to-coincident channels channels (L/C/R). The question then becomes- "how wide" and secondarily -"which pattern", both of which get me thinking about how the nulls of super/hyper/fig-8 patterns are oriented and what influence that would have. Here's a hypothetical- should we use crossed fig-8's and point the nulls just outside the PA stack on the opposite side? Anglewise with the 426, that would be pretty much the opposite of what you have here (at 55 degrees inclusive), definitely requiring the forward facing supercardioid to compensate.
Or perhaps best to split the difference between those two, using a traditional 90-degree inclusive angle for the 426 in hypercard, supported with the center-front (and possibly rear) facing microphones, which also provides redundancy of the 426 on its own as purist coincident stereo. Fun to think about the possible variants to toy around with.