How about to use the Onkyo Audyssey for the x-over and get (+-$100) the basic MiniDSP. Since the Onkyo does the x-over, you can use the 4 channels of MiniDSP with 4 parametric equalizers, so you can equalize, delay etc each sub separately.
I can’t remember if the 818 has xt32 with subeqHT, or just MultEQ. If the latter, than it can’t do two subs individually, or a pair in front as “sub1” and a pair in the back as “sub2”. Which won’t really work if the room/LP/sub placement aren’t symmetrical anyway.Currently upgrading my subs ( from (2) 12 and (2) 15 , all different from each other) and looking for some bass management of some sort - and only have a the onkyo 818 w/ audyssey - will the mini DSP be a better way of getting all (4) of my new subs ( SI 18HT) to work together? and if so which mini DSP to get, went to their site and there is lots to choose from( I don't know much about these , so hoping someone can direct me in the right direction) or is the audyssey that the Onkyo has with it good enough for this? subs will be in 2 pairs 2 in front and 2 in back.