Hi Guys,
My name is Trevor and I'm with SSV Works. It looks like you got the questions sorted out while my account was getting activated but I'll answer a few just for clarity.
First off, thank you for positive comments on the new enclosures. Our design team did a great job and all new SSV Works parts will have a similar 'feel'. I'm super excited about the new slingshot parts we've developed and they're coming fast. I think you guys are going to love 'em.
OK, so... car audio is crazy. As Rab pointed out, 6.75 is the new 6.5. However, to not confuse customers almost all companies refer to them as 6.5's still except for occasional marketing claims.
What matters is the bolt circle (screw hole pattern) and the clamp diameter (widest part of the speaker basket behind the flange).
Most old marine speakers were small and slow to adapt the 6.75 theory until recently. However, Rockford made the RM0 and RM1 marine speakers specifically to fit in our enclosures. The JL audio MX series speakers also line up and fit perfectly. Our favorite option is the KX series marine speakers from Kicker and they also fit and work perfectly. And, the best option is the new SSV Works powersports marine A6 (available in 3 weeks) as it was designed for open air powersports vehicles and is a perfect fit and style for the sling. ALL of those speakers listed above are ARE referred to as 6.5 and not 6.75 by the manufacturers... Even the JL Audio CS speakers you mentioned are actually called a 6.5" speaker by JL. You posted a pic from Sonic Electronix that called them a 6.75" because they do a great job of their own write ups and photos instead of reposting mfg. info.
Hope this helps, please let me know if there are any other questions.
Trevor