well, for a start, what sort of numbers were the 4 strokers putting up?
Don't discount install in what kind of sound you or your friend are making.
I just pulled a pair of 15" L7's out of my car because they were only putting up a 143.3 with 1kw a piece in 9 cubes, while there's people putting up 150's with a single 10" L7, but my peak was only 40hz.
But what sort of numbers were you putting up, just for some reference of what exactly, loud, is to you?
And i'm not trying to beat around the bush, big numbers are doable with just about anything, but to put up big numbers, while still tuning low is going to require some serious equipment, so what's your idea of loud, and what's your idea of deep?
If you want to tune anywhere in the low 30's while hitting 50's, i don't think those pioneers are going to do it. You're gonna be looking at things more in the category of solo-x's and xxx's to hit those numbers down low.