The Sub enclosure sits approx 18mm out from the wheel arch, so for the Titan, based on the PDF, the 1300 seat base is 860mm wide, so if it is fitted center of the van there will be a gap of approx 185mm to wheel arch. which is fine as we only need 18mm. However you will likely fit not center so more to near side. you need to know how close to the wheel arch teh bed will sit. This is irreevant iif the frame is further forward than teh psub enclosure, but I cant see form the drawing where this will be.
The height is 35omm to underside of seat, the sub is 330mm high so it has 20mm clearance.
Check with Titan where their seat frame sits in relation to wheel arch, if more than 18mm then it should be ok. However bear in mind the sub normally has a grille on so this will protrude out past the enclosure 20mm ish depending on sub model.