Need opinion on sub choice

I have a question about some subs for car audio. I have experience in the field but I'm not near as qualified as some of you in here. I am thinking of changing my sub in my truck. I have plenty of room for the encloser. Right now I have about a 4 cu ft vented encloser that's tuned at 32 hz running a Eclipse Titanium 15" sub. It sounds incredible. I remember when I heard it the first time, 3 yrs ago, I was astonished by how clean & loud it was. But after time, I want louder & cleaner. Always happens. So the options I have come up with are 3 subs in total. One of the main reasons for these picks are cost. I should not be spending any money on audio stuff but I do. Anyways, the 3 subs I've chosen are: Eclipse Titanium 15", Adire Tempest v2 15", and Sundown SA15. I already own the Eclipse & Adire. I've always liked the Sundown. Like I've said earlier, my setup sounds great now & it's running the Eclipse. But wondering if I could do better. I dont want it for sq or spl, I want it for both. I can change encloser design, if needed but the box I'm using now is well built and tuned right. Sub is being powered by an old school alpine monoblock amp. What do you guys think? Should I just leave, well enough, alone or try to do better with different sub or box? Thank you in advance.

My current system:
Alpine INE-S920HD Head unit, Audio control Four.1i EQ/Line Driver, Alpine MRP-M650 Amp (sub), Alpine MRP-F250 (highs), Eclipse 15" Titanium sub, Focal components (front & rear).