And in fact it is even more useful for HP side since you can erase the bottom end of a track and even fade out all the hi hats when going for the extreme setting, no need to use both a filter and the channel eq. Again this just feels convenient after I've gotten used to it.
EDIT
I dug up one device and had a go.. it sounded like this, to be honest
Hi-pass : let's some high frequencies through, my bad
Lo-pass: not a full kill but goes VERY low, you barely hear anything with headphones set very loud
So I guess it's just that the Kaoss Quad goes way low so it feels as if it killed all the sound. Cross doesn't go nearly as low since low pass all the way down you still can hear music even with laptop speakers.
So, I was partially wrong about the whole thing. Carry on!