id get a 4x50w rms amp for the speakers. The speakers would only blow if sent a clipped signal, more likely to happen when running off the head unit. An amp should hopefully allow the sound to be loud enough for your liking without clipping the signal.
SPOT ON bezz..... i was waiting for someone to say this :thumbsup:
Anyways yeah i'd get a smallish amp (50-80wrms) to run those front speakers, and set the high pass filter to around 70-80hz. You'll get a whole lot more volume from them that way.
Also 1991_Vn2nV, humans (with good hearing) can only hear to around 20khz.
In all honesty i would sell that monoblock and get yourself a high power version, monoblocks are designed to output the optimum power at low impedances, buying another 400w monoblock would only leave him with the option of running each one at 4 ohm, basically this means he'll only be giving the subs the same power, just using two monos putting out half power.
Get yourself a 800-1000wrms (at 2 ohm) monoblock, im sure you wont have any issues with bass then