I'm a bit surprised to hear that the seat shouldn't vibrate. Maybe the size of my enclosure, the sub's power capability, and running over 420 watts RMS to the sub does it. It can vibrate all it wants, it doesn't bother me because I don't sit there...my GF's Mom would tell you otherwise though...
ebock, if seat vibration is going to be a problem for you, try looking into a downfiring enclosure. They're supposedly more efficient at reproducing bass as well...
Jerry, you're right, they need to be broken in. When I first heard the sub in action after it was installed, I wasn't very impressed. I thought that maybe the enclosure wasn't right for it or something. But after playing it for a month at normal listening levels, it broke in nicely. Based on the size and sound production, this is the most efficient sub I've owned to date. This particular model from pioneer according to my research prior to purchasing it, had the highest power ratings in it's class. You can't really differentiate the sound from a conventional size sub. If you're in the market for a flat sub, these won't disappoint.