i had 2 10s recieving 200 watts each in my old car, i know how loud it is. trust me, depending on the person, that is plenty. you can hear it and most definetly feel it. he says he cant even hear it. there's a problem with something other than power (which doesnt give more spl/volume/loudness).
i know a guy in michigan that can take 400 watts, a custom box, and get 140+ dB's out of it....just to prove to you that power doesnt give more spl. though, power is a factor in spl....the general rule for power in car audio: to gain 1-2 dB's, you have to DOUBLE the current power. that means that even if he sent 800 watts to the subs he has now, they might blow of course, but more importantly, they'd only be a few dB's louder at most.
again, check your wiring.