Don't have any 600 ohm to test it with, but I would think it might not have the power to drive 600ohm properly consider it doesn't have a lot of gain (more important) or current (less important). Since it is on its way back to JDS, you might want to ask John if there is room to increase the gain or not.
update: just checked Xnor's chart here using DT770-600 as a model (they have similar spec), and the C5 should be okay.
I might try and see if I can't resell the C5 and my AKG K550 [since both where to be used together] and attempt to find a used iBasso PB2. I'm getting an HM 801 as well, so my next question [and it's TOTALLY out of the budget for this thread sry] is how does thebalanced amp card for the 801 work? DO i need a special interconnector to connect it to something like the iBasso PB2