Best small box requiring 15”s?

Been looking to upgrade my setup even more by buying new subs to better fit my enclosure size but i’m not sure what to get.

I currently have 2 audio ace rebellion 15”s on a 12.5k and it’s loud, but i want louder. i think they require a bit more airspace then i have, (6.4 net total) so im looking to get new subs that only really require around 3 ish cubes per.

So my question is, does anyone know of some super powerful 15” subs that’ll fit my small box size? budget is probably around 1k per sub