beenjammin, it's not primarily the Jecklin disk itself that calls for a mild high frequency lift in the microphone's free-field response; it's the miking distance.
Practical, professional-quality omnidirectional microphones aren't omnidirectional at high frequencies because of their physical size. And omnidirectional microphones at normal, audience-type miking distances pick up only a relatively small proportion of direct sound as compared with reflected sound. Before reaching a microphone that's not pretty close to the direct sound sources, reflected sound has almost always undergone absorption that affects the high frequencies more than low or mid frequencies.
Thus if you choose a capsule like the Schoeps MK 2 for recording at a distance in a reverberant space, you will get a dark sound; its super-flat on-axis curve (how flat can a curve be, and still be called a curve?) doesn't apply when the predominant sound energy is coming from off-axis.
I've attached several graphs to this message. One shows the approximate difference between the on-axis response of a small (Schoeps-sized) omni and the general, integrated response of the same capsule to sound arriving from random directions in three-dimensional space (a/k/a the "diffuse-field" response). Omnidirectional microphones have more of this difference than any other type except shotgun microphones. (Conclusion: Shotgun microphones designed for placement fairly close to the direct sound source aren't a good bet for more distant recording in reverberant environments.) (Oh--that's every shotgun microphone ever made by anyone.) (Oops.)
But yeah, in most Jecklin arrangements, the capsules don't face the direct sound sources. So that's a factor, too.
> I guess one of the MK2 variants -- h, s, x -- would be ideal, but I read a thread on Gearslutz where someone claimed to have it on authority of a Schoeps engineer that the right in-software filtering could render any two variants acoustically identical.
The MK 2 S would be my suggestion. And yes, all four of Schoeps' omni capsules have essentially identical polar response (see attached graphs) and all other parameters, except their high-frequency response. Thus if you equalize them appropriately, they become interchangeable with one another both on- and off-axis. That's not "inside information" nor is it a situation unique to Schoeps.
(note that in the attached graphs, the old MK 3 is still called by that name; it was renamed to "MK 2 XS" a few years ago, but it's the same capsule. It has approximately flat response when placed in a diffuse sound field, i.e. where rather little of the incoming sound is direct/on-axis. This makes it a special-purpose item in terms of modern, stereophonic recording, but it still has some applications today.)