This interests me even though I have no need for it. Am I alone in this? haha
It sounds like, with nothing plugged into the sidechain / send/return, the HPF knob is a hardware version of the same control in my Darkglass Hyper Luminal, a HPF that governs what the compressor "sees" when it's doing its compression on the main signal (which is not actually filtered). I can work with that.
Now, is it possible to insert an external signal into the send/return that doesn't have capacity for a "send" side? For example, if I wanted a microphone on a kick drum to trigger "ducking" in the compressor in the style of a studio sidechain?
Or, for that matter, a send to a mixer that's taking that send and also the kick drum, and blending them into the return so that the pedal is compressing according to a mix of bass and kick. Is it possible? Would it not be rad if it is in fact possible?