neo2015

The @Ricci CKRAM would be an excellent choice, but it uses a 21" driver, and will probably be heavier than what you are looking for : https://data-bass.ipbhost.com/topic/868-riccis-ckram-subwoofer-and-files/

Is there a way to adapt the CKRAM to use a 18" driver?

Given that you're from Spain, and want a subwoofer for music, i would suggest two sealed 125L boxes with a BMS 18N862, you should be able to get each for about 600€/650€ in Spain.

I've modeled the B&C 18" ds115 - 8, and unless i've made some error, it seems that it only needs very small box to achieve a 0.707 alignment in a sealed box.

That's a major benefit when comparing with the other similar drivers.

Which would you choose between this B&C (or the 18" version) and the BMS 18N862 for a sealed subwoofer in a 125L box?

Are they more similar than different?