I've pretty much decided to set my M-S rig at the front of the stage, and use an omni for the Mid mic. Bass is likely to be pretty heavy thereabouts, and I'm going to build a low-cut filter to use inline with the mic cable.
I can't find information on how steeply the average cardioid mic rolls off bass when proximity effect is not a factor . . . I'm guessing 6dB/octave (one-pole) but don't know. I suppose my filter should try to mimic the low-end response of a cardioid in the farfield. But steeper might be better.
So, what would you suggest for the low-cut filter design: one- or two-pole (6dB/oct or 12dB/oct), and where would you suggest I put my -3dB point.
Hmm. Good question. Various cardioids roll off at different points depending on their design intent. Complicating things is proximity effect.
Let me think out loud a bit are throw some data points out there and see what floats..
Seems I'm commonly adjusting subwoofer heaviness around 100-120Hz for amplified stuff. Yet boom and room-thickness issues range from 80 up to 300Hz or higher.
Dipole rolloff is 6db/octave, but the -3dB point in cardioids seems to vary anywhere from around 100 to 500Hz . The Microtech Gefell cardioids I use roll off from about 200Hz down, the MG supercards I use are actually flatter down to a lower frequency than the cardioids. Both have low-cut filters on their amplifier bodies spec'd as "-15dB at 60Hz" but I never engage them.
I pulled up the specification sheet for the Sure KSM 141 at random (since they are good at listing specs), and it has 3 low frequency contour options: Flat, –6 dB/octave below 115 Hz, or –18 dB/octave below 80 Hz. Those are probably intended for proximity compensation and wind-noise reduction.
The DPA 4098 supercardioids I use roll off at 6dB/octave from about 500Hz down. That would seem to preclude them from taper use, but I always use them in arrays in combination with omnis which fill out the bottom end and in all situations they provide a nice clarity without getting bogged down. The bass from them is smoothly attenuated but still there, and can be pulled up using EQ. Actually at bass-heavy events they can sound quite balanced on their own without EQ or the addition of the omnis. Since your SBD feed will be providing bass extension in a somewhat similar way to the omnis in my setup with the 4098, I think in this situation a similar rolloff to the 4098s may be appropriate for your omni Mids. At bit of excess bass rolloff is going to be safer than less given that you have no EQ ability on your mic pair, and since the SBD feed you will be mixing with is presumably full-range.
Given that, I'd probably shoot for
-6db/octave from say around 400-500Hz. I think a single pole filter slope in combination with a relatively high corner will blend best with the SBD source. If you didn't have the SBD feed that could be more low-cut than you'd want, but then again it might not as things often get very bass heavy at the front of the stage. For spicing up the SBD with some stereoness, stage-presence, and audience reaction, you really only need the mid-range and higher stuff anyway as long as you have enough bass extension from the SBD feed.