Subwoofer

Multiple distributed subs give even, not accurate, bass at the LP.

All-Pass phase controls operate at only a single frequency with varying degrees of shift across the passband.

See 

I used successfully use a single sub for about 15 years.

With only one sub, it's important to remove the sub range from the mains. AND have sufficient drive from the sub.

Most subs have inadequate controls to integrate properly. You must have variable crossover frequency and slopes, continuous phase and polarity inversion.

You can get away with a less capable sub by setting the controls to minimal, e.g. highest crossover and using miniDSP or other capable crossover. I do not recommend using the miniDSP internal A/D so either feed it digitally or only use it for the low end only, feeding the mains with a passive analog XO to roll the bottom octave @ 6dB/octave.

See ieLogical SubterraneanHomesickBlues. There useful links for room analysis and other information.

You left out the single BEST brand of subs to consider for two channel: REL.

Once I would have agreed. In twenty years, I’ve yet to hear decent demo of a REL product, even in $100k systems.

IMO, REL gave up engineering for bling. Most of their line lacks adequate controls to integrate properly.

Far too many conflate more with better.

A well executed sub with adequate controls* properly integrated with the mains improves the listening experience out of all proportion to the frequency response.

I ran an Audio Concepts Force with Spica TC-50 XO @ ≈85Hz rolling the lows first order from the Spicas and third order inverted in the sub.

A composer [real music with real musicians in a studio] once remarked "Those little speakers aren't putting out all that gorgeous bass?!?!?!?!?"

* multi slope XO, phase, polarity, gain. LFE input is nice if it does double duty in HT.

AND drive it low level from the pre. You don't want all those connectors and extra wire in the power amp / mains circuit!

^^^ Read Floyd Toole's book.