The Wet Sounds HT-4 would be a perfect amp for 2 pair of XS-650, but i'd like to clarify its output.
With a single 4 ohm speaker wired per chnl, its output would be 110W to each speaker. With a pair of 4 ohm speakers wired in parallel to a chnl, the 2 ohm output would be 150W, with each speaker netting 75W. This route would allow for a 3rd pair or a sub to be added later.
75W rms is still respectable power for a 6.5" in-boat, but if one wanted to really push the XS-650 to its potential, the HT-2 would deliver 300W x 2 @ 2 ohm. Thats 150W net rms to each speaker with 2 pair driven by the HT-2.