Wattage doesn't matter. Amps per rail are what counts. The key with that power supply is that it has 2 12volt rails, each rated at 32A individually, or for a combined maximum of 56A)
It doesn't look like Athena has a manual online, but you need to check out what 12v1 and 12v2 actually power. IF the hard drives are running off of the same rail as the CPUs (ie: both are on 12v1 and 12v2 is dedicated to the PCIe connectors (not an unusual situation), then you are really pushing the supply to the limit (and beyond) due to the spinup current required by the drives.
Side note, PSU for server's running actual RAID systems can generally be rated lower than what is necessary for unRaid. This is because under a normal RAID system, the drives are running 24-7, and during boot up, you would generally enable the controller to spin the drives up individually rather than all at once (which happens say at the start of a parity check), to keep the current draw at a minimum