In car audio it is normal practice to run amps at lower impedances to achive a greater power output , many car amps are stable at 1 ohm and some even advertise 1/2 an ohm. the idea is to make a 100 watt rated system rated into 4 ohms achieve upwards of 400 watts into 1 ohms so the owner could try to achetive the loudest SPL in the 100 watt class. your best bet would be to bi-amplify, 1 amp for subs with 2 8 ohm woofers parralled and the other for front and rear and avoid using the crappy amplifier sections all powered decks come with. Rockford Fosgate make good sounding, bulletproof amps. Personally I wouldnt run under 2 ohms to maintain sound quality and reliability.

* note...when you bridge a car amp and run it mono into a 4 ohm load each channel of the amp will see 2 ohms.