Well, I owned a 650XL and currently still own two 550XLs.
I am not sure about the 750XL, except that the PU config is different than the 650XL, but........in general.
The core construction of the guitars is almost the same.
The550XL is more plain - dot markers, no binding, etc.
Maple neck through, Poplar wings, which actually work very well on those guitars. Two PUs, one HB, one SC size that is slanted and does NOT have room for the Fenderiish or typical SC flange.
The 650XL has the Shark Tooth inlays, nice binding on the neck, 1 HB 2 SC....same non-flange thing.
It has a mid boost control - I didn't really dig it.
Both guitars are excellent IMO.
Quality is uniform.
Good hardware, including the Floyds.
Sound is surprisingly resonant.
Playability excellent.
I changed out the PUs on the 550s.
I kept them and sold the 650XL (although I kind of hated to) as I was reducing shredder guitars for other types and the 650 had more value.
If you are looking for a neck through type guitar, I think they are excellent choices.
Good luck.......
