More info to add to this thread…
I’ve been using the 21 aggressively in the studio for recording acoustic instruments. What has surprised me most is how tight the capsule really is - it is NOT as omnidirectional as we like to think it is across the frequency spectrum. It really does not follow the behavior - especially in the near and mid fields - that the plots suggest. Just goes to show, we need to trust our ears and not our eyes.
In reality, the 21 is moderately omnidirectional in the treble frequencies, but has maybe a +/- 30 degree range for midrange, and bass is even narrower. The cap is really a lot more focused where we expect to hear musical content, and the top end is close to omni. This to me feels counter intuitive to what plots and traditional Omnis would expect. It also means this is arguably the most demanding cap I’ve ever touched - I’ve had an easier time using my mk2 pair than the mk21.
And that treble response is a huge difference between the 21 and 22 - the 22 is MUCH rounder and softer in the treble. To my ear, it’s closer to a 4 with a bit more bass, and is in the direction of the 2 as far as opening the 4 up whereas the 21 is more in the direction of the 2H or 2S.
I’ve started running the 21 closer to AB, and I’ve liked the results. Traditional NOS in my opinion is MUCH too wide. I think the 21 might be ideal for PAS theory given how tight its midrange and bass throw are, especially with respect to its open diffuse treble response. I need to collect more data when taping to speak further to this.
I think the 22 is an easier cap to get a great sounding recording with. I think the 21 is aggressively demanding and I need to spend more time learning it, but once I figure it out I know the 21 will “put me in the room” in a way the 22 can’t.
I have a mk22 pair coming in the mail this week, I may try to run both the 21 and 22 for Perpetual Groove next week. If I decide to use both in the same configuration, I’ll post the results here. I’m more likely to run my 21 in AB and the 22 in NOS, but the direct comparison is definitely tempting… if not this week, then definitely soon.