You do not have the same type of control over Drum Kit Voices as you do over Normal Voices. This is one of the differences. Normal Voices feature 8 Elements and some 101 parameter destinations for Control assignment. Drum Kit Voices feature 73 Elements but not as many individual parameter destinations per each Element.

DrumKit Voice Elements are far more autonomous, as each of the 73 keys is, technically speaking, a different instrument, where in a Normal Voice usually all (8) Elements make up a single instrument. And while every one of the 73 drum/percussion Elements can be individually tuned, individually panned, and has its own individual HP and LP Filter, you cannot assign a controller to just a single Element's Filters. (It's deep, just not that deep)

What to do:
Nothing prevents the clever programmer from building a "drum kit" using the Normal Voice as a template. There are several examples of this in the Presets, they are the so called 8Z or 8 Zoned Drum Kit Voices. Yes, you are limited to eight Waveforms, not the 73 you would get in a regular Drum Kit Voice.

I've played with some world class drummers in my day, and not one of them had a 73 piece kit (I did record a drummer/percussionist with that many) but most did not even have an 8 piece kit. Point being if you want to have this type of special control (MW or FC controlling a specific drum's Filter) the solution would be to build your own 8Z Drum Kit Voice by starting with a Normal Voice.

Study the Voices with "8Z" in their name, this way you can see how the programmers tackled this very issue. These were all constructed to give a rather unique feel to the drum voice within their genre. You can shape them any way you desire.

Thanks Phil. You actually explained this method to me before and I know how to set that up. I need more than 8 waveforms in this VOICE as it is mainly an FX VOICE. I didn't get the answer to my original question which is:

How come CUTOFF is available on the MOX panel and it works for drumkits but I can't find it in the VOICE edit mode?

The CUTOFF control, as in the front panel knob, does not actually represent a Filter. It is an OFFSET knob (it adds to or subtracts from the currently stored Cutoff Frequency). Turning this knob will only affect the selected PART, or PARTS, if they have a FILTER active in the VOICE.

In other words, if you setup a VOICE and set the FILTERs to THRU (no filter), turning the CUTOFF Knob would do nothing at all to the VOICE. Because the CUTOFF knob is only an OFFSET applied to a FILTER that is in a VOICE. If however, the VOICE has a Filter (a filter can be applied to each Element - that's a minimum of 8 per Normal Voice and 73 per Drum Voice) that filter will be offset. So turning the "OFFSET" CUTOFF does indeed affect all the active FILTERS in the DRUM KIT VOICE if it is the selected program. Each Key has a High Pass and Low Pass Filter - these are not available as assignable parameters within a Control Set. (Only the Filters of Normal VOICEs can be set to be controlled by physical controllers)... How come? $$$ and resources... besides there is a practical reason (first and foremost) beyond the velocity sensitivity parameter, drum and percussion sounds are so quick in terms of duration, that type of control by physical controller is unwarranted (sorry)... but having a LPF respond velocity to brightness makes sense from a musical point-of-view, and since the longest drum sound might not even be 250ms the Velocity Sensitivity to LPF parameter makes the most sense.

If you are constructing a Sound FX kit you can use multiple Voice locations.