I set the amp gains to just before clipping and it is noticeably louder, however there are now more rattles that need to be sorted. As I was worried about over excursion at low frequency I raised the infra sonic filter up until I could hear it affect the volume at 40Hz and backed it off a bit. The cones looked like they were moving a bit too much, but a quick check with the rule showed about 15mm displacement, the Xmax for the subs is 20mm.
So I'm feeling much happier with the sealed box.
Picked up the wire for the components too. 50m of 2.5mm2 OFC should do, right?