I totally 'get it' if the customer even looks remotely unhappy - we try to make some accommodation to keep the client happy. What or how much that needs to be isn't the point.
This technical issue appears to be an operational failure on the part of the BE. While a client doesn't care who is responsible (and clients tend to look at the payee name on the cheque), neither Bill nor a member of his staff caused the problem. If a design shortcoming (not enough rig sounds specious) is evident, a discount on next year's show might be in order; if the client is hiring acts that do not advance the gig or, day of show, need far more than the client is paying for or agreed to, that is not Bill's fault either.
As I said earlier, for a 25% discount there must be other, less obvious factors at play. I also wonder why Bill felt it necessary to perform his Mea Culpa in the forums.