D’Agostino Progression integrated vs Gryphon Diablo 300

The way to do it is to buy or borrow both and compare in your own system. Diablo is often available used, D'Agostino is not.
Most likely different people would choose one over the other even if liked both. I suppose, I myself would have a hard time if had to choose.
Diablo is Gryphon's best effort so far, Progression is not D'Agostino's best effort, that is Momentum integrated. Used Momentum could be the best option, if you can find one.

Separates. What can you get for $16k-$18k new including interconnects and extra power cord ? Probably much more dollars to beat those integrateds. With $45k D'Agostino Momentum integrated, that's different.