18 inch subwoofer vs multiple 12 inch subwoofers with bookshelf speakers

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After thorough research, I have narrowed down my options to two configurations: 1x Rythmik F18 18-inch subwoofer or 2x Rythmik F12 12-inch subwoofers, with the possibility of expanding to two F18 or four F12s in the future.

My primary criteria for the subwoofer system are musicality, speed, transient response and depth of bass response, particularly in the low-frequency extension below 18Hz

For your criteria, you won’t do any better than Rythmik subs (there is no competition). Get two F18s now. Later on, add two F12s as funds permit. I would also recommend the F12G w/ the GR research driver over the F12.

Depending on the processor you have and room layout, you can load up different combinations of subs for the content you are listening to. You could also go all out with 4 of them. The F18 can be mind blowing (!!), f you take the time to integrate it correctly. I was raving about it relatively recently in some other thread. Dr. Brian Ding at Rythmik is very helpful and should be able to answer any questions for ya,

I would also recommend the F12G w/ the GR research driver over the F12.

+1 since music is your priority.

I know the brand Rythmik but have never used them in my system. In general, smaller drivers, 12 inch in your case vs. 18 inch, will deliver faster bass which can keep up with your song.  Larger drivers, in general tend to be slow and likely suitable for movies in home theater set up.  Dual subwoofers are also better than one subwoofer.

Fast bass, slow bass - myth vs. Fact see here! I agree with @james633 😎

Mike