Hey guys how's it goin?
I recently installed two Infinity Perfect 10.1's powered by a 900w Rockford Fosgate amp, and 4 Infinity Reference 6x8 speakers.
Here's my questions...

What is the wattage of the factory amp (non-alpine)?

Can you replace the factory amp with another more powerful amp in it's
place? If not, where should I mount it?

What kind of amp should I get (depending on the answer to the previous question)?

I don't know exactly what the factory amp is rated at, although I can tell you it probably isn't enough to power your four new speakers to their potential.

As for replacing the factory amp, that would probably be the way to go but as far as mounting a new amp in the same place, IIRC there isn't alot of room since the factory amp is so small. Your best bet would be mounting it in the trunk or you can even see if there is room underneath the passenger seat.

I think your speakers are rated at like 60-80watts? Depending on your price range, look into some multi channel amps online or go to your local shop and see what they have to offer. Don't have the time right now to help you pick something out but like I said do some research and find a good multi channel amp that suits your needs (power and price wise).

Hope this was helpful :)