Why are you looking at a 4-channel amplifier to replace a 2-channel amplifier? Based on 2 minutes of looking over Google search results you have quite a nice amplifier.
Those old opti200s are nice amps. But you are comparing a 2 channel to a 4 channel like already said. If your needed a 4 channel then of course. The PPI900.4 is a great amp though.
If you are wanting to replace the opti with the ppi and just run 2 channels I would say it would be a waste of money unless you need more power and active crossovers.
As far as sound quality and reproduction , I have factory front comps as well as comps in the rear doors. I'm only running one set up front right now. Have thought about getting another set to match the fronts for the back thus in the end having 4 channels would be beneficial. Why is it everyone says running comps in the rear is a bad thing ?