At the back of each copy of HCC they offer their Top 10.... subs / AVRs etc. Next to the subs they have a panel that reads as follows:

Using two subs (or perhaps four) should, in theory, bring huge rewards in terms of bass performance. But remember that LFE is a tricky beast to control. Running 2 subs may simply double problems in your room like room boom and suck-outs, where outgoing & reflected bass sounds cancel each other out. If you have some flexibility in where you place subs, trial & error may throw up some interesting results. The acoustic wizard Floyd E Toole suggests one LFE sub to either side of the room may be best for accuracy while placing them in the front corners would be best for overall bass power. Yet there is no hard & fast rule as to what will work best for every room, and you may find that simply by using one (large) sub is preferable as it's the easiest to get sounding right.

That seems to run against the perceived wisdom on here. Interestingly I've been toying with adding a sub (or 2) for a while & had a conversation with one of our PPs recently. I asked if (that brand)'s top-of-the-range sub would be better than 2 of (that brand)'s mid-range subs. "One top-of-range, every time" was the reply.

For completeness, I should add that 2 x mid-range didn't quite add up to 1 x top-of-range, in monetary terms.

Cinema: Meridian 271, 3 x DSP640 (behind screen), 4 x DSP320 (in-ceiling), 4 x DSP3100 (surrounds), 2 x AC12 (interface). 2 x Perlisten D12s subs, Lyngdorf MP40 processor. DT 145" 2.35 AT screen. Sony VW870 projector & S-T lens, Lumagen Radiance Pro.
Lounge: Roon, NUC, Kef LS50 Wireless + 2 x REL Gibraltar-2 subs

Meridian owner since 1998