Woofer suggestions needed for 6L PR enclosure

Hello all,

I'm in the process of designing/making some high end-ish speakers. The problem is that I'm kinda designing them backwards as in I have some cool looking vintage cabs that have 6 liter internal volume so I have to pick my drivers according to that. For tweets i have settled on some Morel CAT 408s and for woofers I was looking at Morel ECW 536 but was wondering If some of yall with more experience could perhaps suggest something better for my goal which is for the speakers to go as low as possible and sound good while doing it. Budget is flexible. Thanks.