I used to debate this for the longest time. However, after a chat with my FC-mates along these lines, we eventually pegged Liam O'Brien for Averill's voice actor. He's got the gravelly ferocity of his personality in spades, especially when he takes on characters like Asura (first clip):
[youtube]O1-3XjB234I[/youtube]
[youtube]O1-3XjB234I[/youtube]