Ive heard the infinity setup on a handful of towers. To take a line from Forest Gump "thats all i have got to say about that".
I have seen the Cadence in person, but it was not an active display. They reminded me of the MTX marine audio. Given a choice between a pair of 6.5" and 8" for a surf setup, the 8" would be the logical choice on surface area alone.
What volume control are you looking at for $150? For half of that, you can get the Kicker ZXM-RLC dual zone volume control with 9V per chnl line level.
Are you looking to not mount the speakers in the factory location, but instead locate them on the cross hoop?
A single pair of 8" HLCDs will give you the wake range projecting and the right ones will also yield nice near-field sound quality for surfing. This would probably exceed your budget though.