I use an NHT x-2 for high pass and an SMS-1 for low pass and digital room correction (DRC). Only the NHT is in the main signal path and I find it undetectable by ear - although YMMV on that one. The combo works very, very well (and, as mentioned, it also adds DRC for the subwoofer which IMHO is probably the most important benefit of all). It is actually pretty reasonable in price (as high end stuff goes) at app. $750 for the combo - plus the cost of sub(s). I suspect that my dream subwoofer (other than downward firing) was pretty similar to yours. I added a pair of very nice 10" Rythmiks and couldn't be happier. So, you see, dreams can come true.
Good Luck
Marty
Good Luck
Marty