Exactly. I thought I was looking for "SQ" and that is why I bought the rsd12d PGs. It turns out that while they have great SQ, they are nowhere near loud enough. That is why I mentioned "clean" as in no distortion but loud. I truly think that is what I'm looking for. I want the subs to hit the low notes with enough authority to give passengers a headache but I also want it to be distortion free.
SQ, SQL, and SPL seem to be very subjective terms but after hearing some different subs, I think the most accurate description I'm looking for is CLEAN SPL. I believe that the L7 (just bought two brand new 15s for $375 shipped to my door) will fill that role nicely with a properly built enclosure.
That said, any ideas for an enclosure? Again, my space limits are 50" wide x 20" high x 20" deep. At 4.4 cubes per sub, that would make the enclosure 48" wide x 19.875" high x 20" deep. Would it be best to go with 4.4 feet per sub or do they need less to sound their best? Thanks.