I would opt for the single 485. It is better matched with the SYN-2 you have. The SYN-2 will do 700x1 at 4 ohms mono. In stereo it will do 200x2 to the PRO 80's. 200 to each PRO 80 is good but they really come alive when you get to 300 to 400. Off a SYN-4 for example. So being with the amp you have, you will be getting the most power in mono. 700 vs 400. Plus on that tower a single will look OK. Some towers it looks odd. But since you have to mount the 80's close as well. I would just go for what works best with amp you have.
midnight, what is the crossover set on? Should be high pass (not full range) but high pass around 60 hz is a good place to start. With larger drivers you can run it lower than most but still want high pass as you dont need tower speakers trying to play 30 hz sub bass notes. Make sure you have the X1 button out for X1. If it is on x10, you will be multiplying what freq the adjustment is on by 10. So 55 x10 equals 550 Hz. Since you have two SYN-2. make sure yoiur switch is on bridged and the rca into the R on each amp. Make sure you are R+ and L- on the speaker side and make sure bass boost is off. Then tune the system and gain.
Also, with the larger drivers like the REV 10 and REV 410. They like some break in time.