FabFilter

I'm not sure how weird of a question this is or how weird of a desire this is to want to do, so... you know... prepare for that.

What I'm trying to do:
I have a sound that has a lot of bass frequency but is not a bass instrument. Kind of like a weird clap or snare sound. We'll call that (Sound A).
I also have a kick drum that obviously has a lot of low frequency sound, and that's its purpose. We'll call that (Kick).

When the (kick) hits, IF (Sound A) is on, I want the eq to turn on and filter out the low frequencies that would conflict with (kick). Imagine like how side-chaining compression works, but working on an EQ rather than a compressor (which would reduce ALL frequencies/volume of (Sound A) not just the part I want).

Is this possible to do with the Pro-Q?

If not, I have:
the C2, DS, G, L, MB, Q2, and R, do any of those have that functionality?

Pro-MB would be your option.

There's also a plug-in called TrackSpacer which is basically frequency sidechain compression.

VolumeShaper4 (and above) can do it as well, for instance using MIDI triggering.

LFOTool is also an option, similar to VolumeShaper, albeit VolumeShaper offers more options in this regard (for instance 3 bands instead of 2).

TrackSpacer looks REALLY interesting.

Something like that and Dynamic EQ would be great features for Pro-Q3. ;-)

Yup, MB is perfect for this kind of thing. Put it on (Sound A), create a band at the frequencies that you want to duck, open the expert panel, select "Ext", send (Kick) to MB's side chain and adjust to taste. Here's info on how to set it up for different hosts:

www.fabfilter.com/help/pro-mb/support/externalsidechaining

A huge advantage of MB for this is the range knob. With a regular compressor, the problem with this setup is that the lows in Sound A will be compressed more the louder the kick is, which is kind of the opposite of what you want: the problem of clashing frequencies is worse when the kick is quieter. The trick is to set a high ratio and a very low threshold, well below even the quietest kick hit, so that the compressor is fully triggered for every kick hit, loud or quiet. This would pretty much squash the lows of Sound A every time, which may sound unnatural, but here's where the range knob comes in: set it to say 6dB and every kick hit will duck the lows in Sound A by the same 6dB. Too much / too little? Simply adjust to taste! Basically you've turned your compressor into a proper ducker without complicated arrangements with additional gates, noise sources and so forth.

Cheers,