[Bruce]

I did a very similiar scenario, except I found a 6dB slope for the bass high-pass to the main speakers to be unacceptable (too much of the same bass from 3 speaker sources: main L&R & sub). I use Dynaudio Contour 2.8 towers (2-way 6dB 1st order xover and -3dB @32Hz) for my L&R mains with an M&K subwoofer.

I found a symmetrical 24dB Linkwitz-Riley high-pass and low-pass electronic xover to do a much better job of smoothly integrating a sub with main speakers. Then, from the low-pass output of the electronic xover (not passive which is mostly used for automobile systems) you can add a parametric EQ to help minimize bass room modes to smooth out and match overall response between mains and sub.

System diagram:

2-channel sources and HT L&R (like diagram above) --> 2-channel preamp inputs L&R --> electronic xover L&R inputs

xover L&R high-pass outputs --> amps main L&R inputs --> main L&R speakers

xover L&R low-pass outputs --> parametric EQ (BFD) L&R --> powered sub