18 inch subwoofer vs multiple 12 inch subwoofers with bookshelf speakers

18” subs are a bad idea.

@mijostyn Yes, except when it is sealed and has the servo controlled drivers deployed by Rythmik (it becomes doable). Yes, it is a tad bit of a compromise compared to their 12 inch offerings, etc, but, it could put you into the infrasonic miracle territory. OP’s use case is both movies and music.

@thxbest , You could also go with Model G22, which is a force balanced, dual opposed 12 inch configuration.

While this is not answering the posed question, I find it interesting that so many are using crossover points above 100Hz, even up to 120Hz. Ive found that 80-85Hz keeps the vocals out of the subs, and it becomes less "directional". I've always used a single.

@thxbest , I would ask Brian at Rythmik.  He was very helpful to me not just with his product line, but also other principles and how to meet my integration and sound quality goals.  He is very straightforward and not trying to just sell you something.   Certain theories may hold true across various designs and manufacturers,  but why not go right to the source when they are so knowledgeable and available!   As much as I love my KEFs, this is what is so great about dealing with a company like Rythmik getting to interact with the designer and owner. 

 I bought dual F12s through Salk.  They seemed like the best combination of depth,  musicality, and authority for my tastes.  A single F12 is pretty loud by the way.  I haven't heard RELs, but these integrate better than any other subs I've had including with my very fast very musical Daedalus speakers.  I'm not sure about your conclusion on " better" mid-bass on the F12.  Seems like your priority is fast musical response where others may prefer the slam the 18 can provide.   The sheer physical size and weight of the 18 drove me away.  I would have went with the compact 15 if I had more priority on slam than speed.  Don't know what frequency you're considering midbass, but I would be surprised if you received a suggestion to use them (12 or 18) to solve many problems above 80-100 Hz.

Below from Brian:

"FM8 will sound more fluid than 12”, but that means its tactile response is also worse. There are customers who prefer FM8 over F12G. It all depends on the priority. FM8 bass quality is closer to electrostatic panel speakers. Another thing to consider when you crossover too high is the location of sub relative to the main speakers. When crossover higher you have to place sub next to main speakers to get proper dispersion lobes and phase alignment.

Our F12 is our reference frequency response for all of our design. F18 may extend a bit lower, but it will be 2hz or3hz at most. If you like fluidity type of bass, F12 is best and E15/F15 is next and F18 is last. Large diameter drivers do sound more authority in bass, but some may interpret that as boomy."

@deep_333 , Multiple 12 and a little DSp will do exactly the same thing with less distortion.

18" sub. That when I go out to the club.  I'm a little too old for that.