I hate to offer another way, as you seem to be narrowing down options, but, in my opinion, using the Vandersteen subs would be best way to integrate a subwoofer into your system.
The use of a 1st order high pass filter doesn't introduce any delays in the time/phase of the signal. It also lets you amplifier do more by eliminating the demanding lower notes.
I have used other subs in my system and they just don't integrate as seamlessly nor easily as the Vandy subs.
The main constraint is that you need to buy the crossover (either fixed or the M5-HP which allows you to change impedance load from the amplifier). Older models like the 2w/2wq can be had quite reasonably used. The new Sub 3 has an equalizer built in, and can be tuned to the room.
Bob