Any sub in the wrong box will sound sloppy, i bet those 2 15s were in a pre fab box and each had like 1.5 cubes IF that. Stick to a single sub setup and weve gave you your $300 option already.
Post a wtb thread and get a used amp. $100 maybe even less i got an alpine 500w for $50 shipped a few months back
Sundown ev2, or hds200 12/15 $100
Knu koncepts witing kit $50
Leaves you with $50 for your box which a piece of mdf is $36, liquid nails is like $3 and home depot charges like 50c per cut and like $5 for a box of screws look at that you just got uourself a setup for $300!