18 inch subwoofer vs multiple 12 inch subwoofers with bookshelf speakers

I am a sub system user for 20 years now and have tried a number of things. One subs is very poor sounding. It “can” work for one seat but two subs is so much easier to integrate.

if you want infrasonics (bass below 20hz) the 18s will be much better than 12s. But almost all music is filter at 20hz. For movie and music 12s are 100% fine.

as for integrating with small 5.5” drivers… it will sound fine with either size until your speakers run out of steam at which point the subs will take over the sound signature. Just come down to how loud you listen. I would guess at 95db your speakers are starting to compress a bit. There is a reason theaters use compression drivers and or line arrays. 
 

another way to look at it is which sub plays at a higher crossover. Low inductance light drivers can play very high and a crossover higher than standard might actually help you out. Sadly I fine anything over 80hz is pretty locatable though so sub placement would need to by right by your mains for higher crossovers. 

Based on the linked measurements the LS50 takes a big dive by 96db in the bass. If you want to play above 90db-ish you probably need a crossover of 100hz and a safer bet would be 120hz if you really wanted to rock out a movie. So maybe the question is what sub plays the best in the 100hz range. Food for thought. 
 

https://www.erinsaudiocorner.com/loudspeakers/kef_ls50_meta/

In your case, if you can place each in an ideal location then do two.

I don’t believe that smaller = faster, at all. That’s a misunderstanding of what really happens, however if you are flexible in the sub placement so you can smooth out the room response, two will give you the better option.

Also, consider bass traps in the corners and a miniDSP to equalize each sub separately, and plug the ports. Set the subwoofer as high as your measurements say you can.

The miniDSP site had application notes for EQ'ing multiple subs in a room.

Check out the AM Acoustics Room Mode Simulator, it will help you place your speakers and subs outside of the worst and lowest room modes.

Of course, another problem often faces is that a reflective room with all parallel surfaces will be bright. Treat it as needed first, that will help the bass bloom. Then you can set your subwoofer level correctly. Don’t forget the ceiling between listener and speaker.

Bass becomes directional due to it’s overtones, thus bass imaging is possible

A stereo pair of subs, adjacent to the fronts, no port, if so, front port only.

One thing I forgot to mention, specs and measurements like the one's shown above don't mean crap when you get a speaker in a room.  This speaker seems to have plenty of output to 40 Hz, which is great for the size, but in the room this could go either direction.

I would proceed in this order:

• Place your main speakers
• Treat the walls and ceiling
• Evaluate

Treating the walls and ceiling will often bring out the bass to the point you don't want more.  THEN if you still want more, the dual subs, placed in ideal situations with EQ and bass traps may be your next steps.