System Pricing

90 ST

If they were to go with one amp, it should drop the install price, when i worked in retail install was based on how many amps, not the size/power, or what ever they are doing. Our prices, keeping in mind this was a few years ago and in Canada, were $90 for a basic amp install, and $25 for an additional amp, do to extra wiring, ect...but thats ADDING an amp, they are still only using one amp.
I would maybe look at the XD200/2 and the 600/1 and call it a day. I might also look at other shops if they want to charge you that much to put in one amp...

As for the SQ using 2 vs 1 amp, yes and no, you can get more power out of mulitple amps, as most multi channel amps wont have as much total power to split up. For ex. the company i work for has a 5ch amp that is 38x4 and 250x1, where as if you go with multiple amps you could get up to 1200x1 and 50x2. or in my case i went with a different series that we do and have 800x1 and 160x2.