The 12L5 is rated for 600W RMS and 1200W Max and the 12L7 is rated for 750W RMS and 1500W Max...RMS is what the sub is designed to run at constantly (or "continuous RMS") and the max rating is simply what the sub can handle once or twice, but will blow if it handles that power for much longer. A lot of companies use the max rating as a sales pitch, but most of the better quality audio manufacturers (like JL, Kicker, etc) rate their equipment in RMS. Companies that rate their equipment in max wattage aren't necessarily bad, though.
So basically, the L7 can handle more power than the L5 (kinda like a JL 12w6v2 versus a JL 12w7), and I'm sure there are a few key characteristics that separate the two. You'll want to find an amp that's rated for these particular power levels at a particular ohm load (@ 4-ohms, @ 2-ohms, etc), depending on what ohm sub you want to use (4-ohm, 8-ohm, Single voice coil [SVC], Dual voice coil [DVC]). How you wire the subs to each channel determines what ohm load you're running. You'll need to research this if you don't know already.
Good luck with the new system!
BTW, I found the specs on Kicker's website and I know absolutely nothing about Kicker, their L5's or their L7's. A search would have easily answered your questions. Not trying to be a dick or anything, just saying. Once again, good luck!
Last edited by c00lkatz; 08-30-2005 at 03:06 PM.