I have 4 JL audio 6.5" marine speakers in mine all amplified by a Kenwood marine amp with a Kenwood head unit. Two in the sound bar, two in my custom trunk box that also houses the amp.
What I like about the setup is the volume is insane and crystal clear all the way through. With the top off, the sound is truly impressive. Subs don't work that well with the top off which is why I steered to an extra set of marine speakers instead of subs. I truly don't think there is a better quality speaker on the market than JL audio's marine lineup, they are built very very well. That being said they might be the most expensive. I've put in a lot of sound systems for various vehicle, boats, etc, JL is the best IMO.
If you mostly leave your top on, a sub paired with nice round speakers is a great choice. The one you have in your post would be a nice choice. I would consider getting a single amp to power the speaker and amp to simplify things. Depends how far you want to take it.