I probably wouldn't shop there anymore. An L7 takes 1000w rms and I'm not sure about Kicker, but manufacturers often use thermal power handling with ratings. It'll take up to at least 1000w before it will heat up enough to start anything. This is also for extended periods of time at that power level. Unless you crank your music all the time, there will be no problems. I've heard of people running 1500+ daily and haven't had problems.
If what he says is true, then why don't comp vehicles combust the secound they are turned on (100000w flowing)?
Mickey, sometimes depending on your enclosure design, a sub can reach is maximum excursion before the rated power limit. When that happens, you won't get much benefit with increased power. I think your car audio owner was exaggerating a bit with his statement. 600W of clean power may sound like a lot, but in bass world, it's average. As long as you supply undistorted sound, most subs will handle the max power and more without overheating/breaking down.