I think 2 of those kicker 15s in a good box would sound louder, deeper, and cleaner. NOW I KNOWWWWW people around here hate Kicker but lets be real here prefabs ****. I've had 2 CVR 12s in a kicker prefab before and the box was fallin apart and ****.
Now, I don't know if you can get 2 kicker 15s for your budget.
Cone area wise, either 3 15s or 2 18s would be louder than 2 15s.
Idk... put 3 of these in a box ported to 31hz? I just went on
Sonic Electronix, went to 15 inch subs, then put the sorting to lowest price first and scrolled until I found a sub with a good rating lol.
Rockford Fosgate P1S4-15 15" 4 ohm Punch Stage 1 Subwoofer
And put them on a good 1K for now.