u want stupid low lows? Then 3 cubes a sub in seperate ain't gunna do it, not for 15's IMO. it'll be fine for mid 30's and up , but low 30s and below Doesn't seem like enough sapce for them to go stupid low, at least audibly.
Put em in shared enclosure, or at least 3 shared on both sides, 10 cubes a side
I wouldn't wall it but that's just me. I always think it looks cooler subs up and just more practical. Sounds better IMO too.
If u want to wall it go for it, but give dem subs more then 3 cubes lol.