If your single mtx is loud, and good sq, just get another one!
I've owned l7's before, and they're CRAZY loud, but with MOST subs, l7's included, sq and spl are both obtainable, but at the sacrifice of the other. Most people complain about l7s' poor sound quality, but that's mostly because prefabbed and recommended enclosures for them are made to purposely be peaky and loud at shortened freq. ranges for performance, over SQ.
Kicker's a great speaker, but it sounds like your current setup is what everyone's really looking for, the holy grail of car audio, if you will. Hehe. You've got great sq, and it's already LOUD, you just want more, so i'd just reup and double your current setup.
To clarify, kickers are great, but if you scrap your current setup and go with a completely new sub/amp/box setup, you could never find that perfect harmonious balance again. if you just double your current setup, you already know you'll like the sound, you'll just have more of it.
>>and i step aside to open another 40 of mickeys.