On the X1800, the problem occurs in iso, which is a summed crossover network, at most crossover settings, and also in EQ mode that obviously isn’t affected by the crossover Utility settings because this other mode is just a fixed-parameter parametric EQ.
On a normal parametric EQ, the amount of phase distortion increases as you move from the zero for each knob, while an iso has a fixed amount of distortion. It’s important to note that both modes of the tone control knobs do nothing to the sound on the X1800 when the knobs are all zeroed on a channel… even the iso mode. The iso completely turns on for a channel when any of the knobs is moved from center even the slightest. The iso bypass is a canned, artificial trick the X1800 does in digital and is not something that can happen on an analog mixer without a special bypass button or switch. If you tweak any of the isos out of center on a channel on the X1800, you will get the constant amount of group delay phase distortion on that channel as you would on an analog iso. You may or may not be hearing this, but it’s worth mentioning since it is a novel (and I think welcome) feature of the X1800 and has a measurable effect on the sound.
At 100hz and in iso setting, that crossover point is so low that you’re mostly cutting the lower bass even though the rotation curve (think joystick curve, not the crossover curve) is very clunky to the left – the reason you can sorta kinda not overdo the bass cut with the slightest motion to the left when using 100hz in iso mode even with the crude curve. That’s obviously an imperfect mitigation, as you’ve discovered. I have a feature request on this topic:
I would also suggest this problem may be related to the sweep effect deadzone issue, which tends to usually occur to the left, as well. Hard to be certain.
The left side of some (or all) X1800 pots’ digital curve models that have a 12 o’clock center notch might be commonly messed up somehow, like perhaps the right side is correctly log curve while the left side is inappropriately anti-log? That’s a guess, though. It’s certainly worth InMusic double checking it. Obviously some of the dead zones on the pots, like the sweeps, need to be slightly bigger or, better yet, adjustable, especially if we ever get a rotary mode which would require adjustable dead zones for the use of the sweep FX on the faders. I don’t notice any dead zone problem on the tone knobs, though, as I can definitely see the isos turning off and on properly with a scope display and test waveforms. With two X1800s, that’s 24 tone control pots that don’t have a single dead zone issue on any of them on my end. So that’s a positive.