[Wayne A. Pflughaupt]

I guess you mean 8" subs, not 18? I don't see any 18" subs on Pipedream's web page.

Also, about the limited low-end of this sub -- first of all, I only see it on the left sub (the right goes all the way to 20Hz), but my room is asymmetrical.

Ordinarily I'd suggest stacking both subs where the right sub is, but something tells me you aren't going to want to do that...

Off the top of my head, I see no compelling reason to ditch the outboard crossover. It has decent enough specs, and it does have some built-in EQ specific to your subs, according to the web site. Still, as long as the Onkyo is providing bass management, you could just as easily go with that and lose the crossover. The Behringer could accomplish any EQ you need. You don't have to be worried about duplicating the stock crossover's EQ. Just equalize with REW like you would any other subwoofer. The only caveat would be to EQ with caution, given the little 8" drivers. Specfically, if they're not getting great extension you're probably better off just living with it than trying to force it with severe low frequency boosting.

I know this isn't your typical HT setup, and my room presents numerous challenges... my room is asymmetrical.

What kind of challenges? I've typically found asymmetrical rooms to be easier than the shoe-box type, at least acoustically-speaking. Optimal placement options can be a problem, though, depending on the layout of the room...

Regards,
Wayne