Welcome to Help the Ijit, where experienced people give help to (or wickedly mislead) the clueless newcomer!
I am experimenting with using a MiniDSP "2x4" as a headphone processor (cross feed, delay, EQ, etc.) I have found an "off label" use of it -- I have found that, with a Y-connector, you can combine two of the outputs into a blend of your choice. No mixer needed -- it works fine with just a Y-cable. For example, the L signal plus a R signal EQ-ed and delayed to your choice. (And of course similar for the other channel.) This is the basis for many simpler processors. Also, you could probably use the "invert" setting and make a Speaker processor that would be similar to the old Carver "Sonic Holography" or a variety of similar schemes.
Early listening shows much promise. It definitely changes the sound. What I would like now, is can someone point me to proper EQ curves to try?
I am not trying to make a Smyth Audio Realizer, but I am using an ancient Stax Lambda Pro for my cans, so I am on the path sort of!