I work at a retail car electronics store and we are informed to recomend having amps that run the RMS as close to the sub RMS as possible. a little over is fine too. with subs you're more worried about underpowering them, the coils won't be able to move the cone back and forth fast enough there by either frying your amp or sub which nobody wants.
however I have read that you want the RMS of your amp in the uper third of your sub RMS... so anything that is 2/3's (or more) of what your subs RMS is. but you don't want to go to strong either the less distortion the better... a sub can still play great and loud without being distorted.
that being said... personally I would try and match the RMS of the amp to the sub (personally I'm planning on going with a kicker 600.1 with an L5 12") RMS on that L5 is 600 watts, and kicker under rates their amps (it'll be closer to 650 watts X 1 at 2 ohms) however like 97teg says your box and electrical system of the car can also make some differences.
as far as over powering the subs you just have to listen and make sure the subs aren't distorting. if they are make sure the gain is set right (you want it to correct for whatever the head unit preamps are shooting at the amp. make sure it's around 4 volts at the connection from the RCAs to the amp. then turn the bass boost down (if any) just turn the sub down a bit so it's not distorting baisically and you shouldn't have to worry about any peaking or clipping.
the Type S sounds nice we used to have them here, but my fellow installers swear by the type R... if worse comes to worse you could sell the amp you have on Ebay or something and invest it into another amp.
whatever sub amp combination you decide on, make sure to give them a breaking in period (don't blast them right away I've seen so many fried subs cause of that... and it's not pretty!!)