I used AU-24s to drive Legacy Whispers from a McCormack DNA-500 without any problems. Lots of current, 900wpc into 4 ohms. My Whispers are pretty highly efficient (95 according to website) and never had any issues, regardless of volume.
Doesn't sound like your problem is the Au24s, but in your setup, as others have suggested. Good luck.
Doesn't sound like your problem is the Au24s, but in your setup, as others have suggested. Good luck.