18 inch subwoofer vs multiple 12 inch subwoofers with bookshelf speakers

I have been using two B&W AWS600 Series 8” subs paired with Harbeth SHL5+ Bookshelf monitors and found them more than adequate. 200W x2 offers more than enough power without overfilling the room with a super boom boom bass punch that can move your furniture. In short, too much can be way worse than not enough. The size of the area you need to support coupled with the crossover frequency and how hard you can drive the monitors is what drives the formula, not just the bottom end.

Not to 'X' foils with Erik, but...imho 'smaller' is faster, but I've played about with trying to pace with the large Heil AMTs'...but I goof about with various combinations to fill the hours.

Currently, the low-mids X out from a pair of 6.5s' into a pair of 10s' for most of their range.  The 'bottom' Xs' down to an 8" self powered sub...

An odd coupling, perhaps...but my space 'x v. y' is similar to our OPs'....except for ceiling 'z', which is 13~14 ft.  And there's other issues that drive me to 'ignore the room' per Linkwitz...

(...still trying to convince our cats to act as bass traps, but they ain't buying it...messes with their sleep patterns....🤷‍♂️🤦‍♂️...)

I've got 2 - 18s', a 12 that's better suited to a 'street boomer', a pair of JBL 10s' that could be pressing into service.... But, have 4 - 8" that I'm considering for a 'swarm' arrangement for my walsh drivers....*s*

Got the means and the methodology    just have to give up that annoyance called 'sleep'....😏👍

Another vote for multiple 12 inch subs. We have 4 REL S510s in our home theatre and it is fantastic 

@thxbest 

There is a video on Youtube that is titled: "Dual Rythmik FM-8 Subwoofers as Speaker Stands?" that is probably worth a look.

There are definitely times when smaller is less likely to wake the dragons, so I'm not saying smaller isn't often just right, but sometimes excess really is excess.