When I had my 2 18's, after I got my 300 amp alt, my subs literally got twice as loud, and I started breaking things. Low notes especially, my lows increased so much I broke my hatch 4 times. If you want the best, run a lot of alt power, so your voltage never drops below 13.8v or so. It'll drop some under a load, regardless. I did run an external voltage regulator on mine. Initially did 15v with my batcaps but lowered it down to 14.8v with more regular AGMS. That's something you might consider, is getting 2 alts and running a self regulated one, and then running a second alt with a voltage regulator. I kept overall higher voltage under a load from having the regulator set up at a higher voltage. So whatever voltage that was factory, say it would drop 1.5v under a load. Well, how ever much I turned my alt up, it would still drop 1.5v. So if you charge at 14.8v vs say 14v, then a 1.5v drop under load still leaves your amps with more voltage. You see what I mean? That was my experience.