I would mount them on the front bar. The speaker is big and if you mount it on the back bar it will stick out alot. If you mount them on the front bar then the grill will be pretty flush with the back bar and look much cleaner. Just my 2 cents.
If you go to the Wetsounds customer installation page there are a couple pics of my 4 pro80 setup on a white/red Reinell with the pro flight swoop tower (mounted on the rear bar). I think, installed on either the front or rear crossbar looks/perfroms good. The reason I chose the rear bar was to somewhat protect the hearing of the passengers on the rear bench.