I have the exact same cans on my zero flex tower. I replaced mine with 6 3/4" Polk 6051UM. I had to drill and tap out the cans. If you look at the Clarion speakers just outside of the mounting screws there are additional holes. These are where the holes for the Polks line up but the factory cans don't have holes there.
I used a #29 bit with an 8/32 tap (you can use the stock screws) but I replaced mine with stainless allen heads to match the rest of the boat.
Also I had to get creative to mount the tweeters but it was fairly simple. I cut off the back of the mounting (cup) bezels (supplied with the Polk tweeters) and then glued the tweeters to the mounting bezels, then I glued the bezels with tweeters in place. They look and sound great. I found the speakers online for $165/set. I also had the cans powder coated to match.
No pictures but I can get some next time I'm out.
Last edited by JPK; 05-24-2010 at 02:12 PM.