Subwoofer for music, how low?

I am looking for a pair of subs for music. I plan on having four if the two can blend with my speakers.

I have the Tekton Double Impact and tomorrow I will get a set of Buchardt S400Mk2.

I have been looking at the REL T/9x but the only go down to 27Hz at -3db. Should a sub not go down to 20Hz?

I live in Denmark so I have fewer options. I really like the swarm sub array but to costly to get it to Denmark.

I am looking for recommendations. It could be DIY or a specific sub.

I am open for suggestions

Merry Christmas