I run a Zu Audio Undertone single sub b/w my M4 Triode Masters..
It is sealed, not portex, kind of a monster in terms of sound projection.
Goes down th 14hz i believe. But Im not a big sub kinda guy, so one sub works for me.
Plus, i only have room for one sub, 13x19ft room
There has been a fair bit of discussion on subs previously
in the Spatial thread thst you may want to check out