So the 3U capsules are vented…since the body is vented also, does that have a noticeable effect on the pattern or frequency response?
Response of these capsules will heavily depend on what's behind them, how large the chamber is, if it's vented or not. Km84 has shallower chamber and no additional vents that MXL bodies have.
That is true, but not necessarily better. The dome which difuses the sound will be placed considerably lower in the tube of the MXL compared to km84. Once again, these things are way more important and will introduce more audible deviation from the original than any change in the circuit of this particular mic. Even if you compare mxl stock circuit versus the km84.
Slot size on km84 capsule is carefuly tuned and is the part of the delay network. Using more slots is in a way the same as widening the holes on the rear backplate of the capsule.
As far as i can see, this capsule is more of a km184 based. It has tighter slots, so potentially even more important.
I suggest using some foam to make the chamber behind shallower, and listen. Record white noise with and without the foam at same exact position and look at spectrum analyzer for differences. 90°, 180° as well. Big part of km84 is off axis sound.