18 inch subwoofer vs multiple 12 inch subwoofers with bookshelf speakers

I am currently in the process of selecting a subwoofer system to complement my bookshelf speakers (3x KEF LS50 Metas with 5.25-inch midrange drivers, 2x KEF Q150) to achieve a balanced and immersive audio experience. Size of the room is (19.5x13x8).

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. Additionally, I aim for seamless integration with my bookshelf speakers, ensuring a cohesive and satisfying soundstage for both music and movie content without overpowering the midrange and high frequencies produced by the bookshelf speakers.

I would greatly appreciate any insights or recommendations you may have regarding which configuration—either the single F18 or the dual F12s (possibly 2x F18 vs 4x F12) would offer better synergy with my bookshelf speakers and deliver a fuller, deeper sound with smooth and fast transient response across frequencies with 50/50 music and movie?