All four keyboards - the 6200, 6600, 7200, and 7600 - use the AHL sound engine. The core samples are the same as well. The 62 and 66 have 48 note polyphony, while the 72 and 76 have 64 note polyphony. The 62 and 66 have ten user memories, while the 72 and 76 have 100 user memories. The 72 and 76 have more tones and more rhythms, and both have 9 programmable slider controls which are pre-programmed to emulate the sliders on a pipe organ, but can be used for many things. The 72 and 76 also have a leslie speaker simulator, with a choice of slow or fast.
Many have commented that all four keyboards tend to be on the brassy, harsh end of things. You can copy a sound you feel has promise into one of the user memories, and modify it to your hearts content. This can make instruments sound considerably softer. You can also copy .AC7 sounds from an external source onto an SD card, and store them in user memory.
On January 25, 2018 Casio announced the existence of a new sound engine, AiX, which would be replacing the existing AHL sound engines in all Casio keyboards (with the exception of their pianos). There is very little documentation regarding the AiX sound engine. What little there is can be found at this link:
http://www.casiomusicgear.com/products/ct-x-series/ct-x700
The link takes you to a Casio page describing the CT-X700, the first Casio keyboard to use the new sound engine.
Hope that helps - Charlie