Kinda torn on my feelings on this, to be honest. Part of me is pretty impressed that Bardic sonic magic can put out that kind of damage. The other part of me looks at the PnP description of 10d6 damage, looks at that 790 figure, and wonders if DDO hasn't completely ruined D&D :/
It has to be a crit. I've never seen a crit higher than 500, but I just have Maximize, not both Empower and Maximize. Maybe that makes the difference?
EDIT: I just realized too that I don't have the Crit AP maxed out either, just two AP into them.