I think the lvl4 xl's could handle the 3500d.
But maybe it's safer to get the BTL fully loaded,the 12 inch subs, you could make a better box for them, then for the 15's.
You can make a box that's 16.5 inch high, 37 inch wide, 14.2 inch deep, that's around 5cu ft.
With a port that is 4 inch wide and 15 inch high, and 22inch long.
Pics:
Don't mind the "BTL 18 inch", it is the 12 inch T/S parameters that i entered, just typed 18, bu ti assure u that it's the 12 inch.