I don't know if this is the answer, but 75 through 89 are the only patches that start with the prefix DX. These may not be merely FM, but literal DX7 patches, and if that's the case, the patches had no filter parameters, because the DX7 had no filter in it. Voice 90 isn't a DX patch, but rather a TX patch, so while they are not specific about what models these various sounds are derived from, it's possible that whatever TX model was being duplicated in the creation of this patch, filter was supported.

For example this has an impact on the majority of voices just playing near the center not high or low necessarily. The menu curves allow me to scroll through them all in real time while playing but typically rotating the filter to max and min and scrolling through the pre-set curves will make a very obvious impacts except for these groups of voices. I think those curves change the low and high as you mentioned and graphically it shows a visible change. I'll test it again on the high and low registers to see if that makes a difference or I could just use the octave buttons. I attempted to get up and down on those curves so I could detect any changes. Many other voices change dramatically so I didn't consider I was doing it incorrectly.

For example #74 is FM Synth Pluck and #75 is DX Brass 1. Both FM voices.

When rotating the filter knob on each and comparing the result, this changes the pluck very obviously but has no effect on the DX Brass no matter how low or high I play or which filter curve I select. Same for #76 thru #89. Almost like the filter option is disabled. #90 starts working again. I have adjusted the EG in my tests to make sure it's not cutting off prior to filter impact. Again as with many other voices filter works normally as you'd expect.

New voice sets on page 15 and 16 the filter knob works but page 17 only voice number 90 works using the filter.

For example DX Strings 2 (pg 17-2) is okay but pretty boring sounding. The real strings is much better. My options for enhancement of this voice are limited to EG and filter and the FM 2 unison and 4 layer (detune and spread) options including the EG and filter curves. So I can dive into the menu and enhance this voice accordingly using these available tools. I can definitely make it sound better or more interesting but unfortunately the filter doesn't work.

The filter control in the menu combines both the cut-off and resonance curves so although I can't directly change them independently I can select a pre-programmed response curve to use or call it a model. It still does nothing for these voices noted. Bottom line is there are several ways to test to see if it responds however It doesn't. It's trial and error but works for many other voices.

So does that mean to be true to the original they disabled the filter capability? I thought a filter could be applied to anything if they wanted it to. That's taking it a little to far I think because allowing a filter is better than not allowing a filter. I also assume that if a voice isn't an FM generated voice (DX is FM) then you wouldn't be able to use the FM features such as unison and detune etc. These voices do allow that feature so why wouldn't it also allow a filter? Must be disabled.

The YC doesn't expose the filter type to the user. Not through the interface (onboard) and not through (documented) SysEx. You get different curves for how Cutoff+Resonance follow your knob twirling and that's about the extent of what you can configure in terms of the filter.

I say this because if each sound can have its own filter (including a choice out of various filter types) then these are hardcoded in the firmware. And if any of these sounds hardcoded filter=off (thru) then of course cutoff and resonance would not do anything similar to what happens when we program a Montage/MODX with no filter.

It'd be interesting to learn more from Yamaha about the filter type(s) used in the YC and if there's any opportunity to enhance the preset content with more activated filters.

I was going to make a similar comment about the DX and its lack of filters (as a possibility for the "why" ). All I'd add here is that without filters how DX accomplished filter effects was by manipulating the operators. You may be able to brighten/darken a sound by using the operator volumes (drawbars in the YC) instead of the YC's filter knob for FM content. It depends somewhat on the structure of the sound (operator "stack" ).

The YC doesn't expose the filter type to the user. Not through the interface (onboard) and not through (documented) SysEx

Not sure if this is related or not but the doc does mention LP Filter as an Effect Type on page 33
https://usa.yamaha.com/files/download/other_assets/5/1376845/yc88_yc73_en_om_a0.pdf

LP Filter

Filter that cuts sounds at frequencies higher than the cutoff frequency. Turn the
[DEPTH] knob to adjust the cutoff frequency, and the [RATE] knob to adjust the
resonance.

Stereo

Curiously that describes using separate knobs for cutoff and resonance which seems to contradict what the 'Filter' section says on page 35

Filter
Filter is a function that changes the tone of a sound by passing or only a specific frequency range and cutting signals in
other frequency ranges. This instrument controls the following two characteristics with a single knob.

P33 also mentions Harmonic Enhancer as a high-pass filter effect type on the same page.

Then on P66 in the SysEx section the effect type is listed as
Addr Hi - 46
Addr Mid - 0
Addr Lo - 2B

And the Lo settings available, include LP filter and Harmonic Enhancer are listed as a footnote

*1: G Chorus, SPX Chorus, Symphonic, 816 Chorus, VCM Flanger, Cross FB Flanger, VCM Stereo
Phaser, Small Phaser, Max90, Dual Phaser, Tremolo, Auto Pan, Simple Rotary, British Combo,
British Lead, Small Stereo, Compressor, Auto Wah, Touch Wah, Pedal Wah, Cross Delay, Tempo
Delay, Analog Delay, Looper Delay, Room Reverb, Hall Reverb, Reverse Reverb, Lo-Fi, Ring
Modulator, Slicer, LP Filter, Harmonic Enhancer

Yeah, I saw the filter effects but thought that'd be a hacky way to do it and maybe the results wouldn't be what are desired. Still, you can try the two filter-ish effects and see how far that takes you.

Yes that's an effect option that replicates or simulates a filter type same as that rotary effect wasn't the same animal as the dedicated rotary module. I'll check that out to see what it does exactly but a true filter impacts the voice prior to it reaching the effects section. My CK has just been shipped, should be here for a comparison soon. It's got over 100 leads and pads besides pipes. I can only assume these surpass the quality of the CP line that wasn't that great compared to YC. They thought further ahead and separated the filter and EG controls out into individual options instead of a shared function.

[quotePost id=121090]Not sure if this is related or not but the doc does mention LP Filter as an Effect Type on page 33
...
Curiously that describes using separate knobs for cutoff and resonance which seems to contradict what the 'Filter' section says on page 35[/quotePost]
Page 35 describes the operation of the dedicated filter/envelope control, which has only one knob. Page 33 describes the operation of the additional filter/envelope controls implemented in the Effects 1 and Effects 2 sections, and those have two knobs.