In theory, yes, you could bridge a single 4 ohm rev-8 on those 2 open chnls. But, Those speakers are not sold as singles, so where are you panning on getting a single?
Sound wise, it will sound like arse, especiall if its only getting a mono signal.
If you only goal is some how use those chnls, then they would be going to waste if you wired up just a single HLCD.
Bridging a sub-woofer would be a FAR better use of those 2 open chnls, or even a 2nd pair of xs-650 in the boat or on the tower. My best advice, evaluate your entire system to determine what part of it is not meeting your needs and goals. Then you can see whats needed to fill that gap and that can be wired to those chnls. Personally, im a huge fan of a sub-woofer to fill in those missing notes that the in-boats wont reproduce. Even if its a 10" sub with only a couple hundred watts rms. If more output from the tower is needed, then replace the pair xs-650 with a pair of Rev-8 and bridge them on 4 chnls for 300W each. Much better use of the amp.