Is an in-wall volume control rated at 60 watts (continuous) adequate?

Just barely is the simple answer. Go for one with a higher rating if you want a reliable system. Though The Director is rated at 60 watts, that is a conservative number and it can put out more power if only a few channels are driven. In contrast to the conservative rating of The Director, the wall volume control may be rated using favorable assumptions. Also make sure the volume control power rating is continuous not peak. The continuous rating is about one-third of peak.