http://www.pioneerelectronics.com/PUSA/Car/Amplifiers/GM+Digital+Series/GM-D8604
http://www.sonicelectronix.com/item_63974_Pioneer-GM-D8604.html
Would it makes sense to still go with the ppi900.4 if he will be using the crossover features from his Pioneer 80PRS head unit, could he just save some bucks and run the Pioneer GM-D8604 above? Sometime around the mid-90's Pioneer lost me permanently on their amps. Doesn't mean anything though, but the sour taste in my mouth still exists. Compounded by being hard to recommend an amp that I personally have no experience with. I haven't actually used a PPI 900.4 either, but have 3 of the Polk versions of exactly the same amp.
And personally, I'd never use ANY crossover on an amplifier. Makes tuning a complete fuckshow. The exception of course being an SSF as unless you have an offboard proc you are almost reduced to having no choice.
So, Pio might be fine. Long term resale I'd doubt in comparison though. To me cost is always buying price - selling price which I'd bet wins for the PPI. In particular if a used copy were purchased.