Hi Matt, Its a little filter unit called the 'Philtre' its got low pass, high pass, band pass and band stop filter settings on it as well as loads of parameters to fiddle with.
i think its a great little thing, with a little practice you can do some great filter sweeps etc. when i saw this a couple of years ago i had to get it. Im not sure if they have been discontinued now, but i only paid £75 for it new. ive always loved my filters and i hunted around for a while trying to find something that would do what i wanted. i looked at the Electrix Filter Queen and had a go on a Vermona Action-Filter but i dont think they're designed to work on your send and return?? i think they're meant to plug in-line with your equipment so you need one for each deck...but maybe im wrong im no expert?
here's a close up if your interested...