If you're really concerned about it, you could mod the amp with a 15A power input fuse or circuit breaker. Then you could replace the cord end with a NEMA 5-15P. Then you wouldn't have to worry about overloading anything -- except for the 15A fuse, which is supposed to fail gracefully.
How would that help? Adding a fuse to the amp wiring does nothing to protect the unknown venue wiring (which is sized to the breaker size not the receptacle size - i.e. you can have 5-15R on #12 wire from a 20A breaker but you can't have a 5-20R on #14 wire on a 15A breaker), and only increases the voltage drop to the amp by the fuse's resistance.
The 5-15P ->5-20R adapter is fine unless you're trying to pass a UL inspection. It will have absolutely no measurable affect on amp performance, and as long as you build it with appropriate wire, will be safe. If there's a wiring overload, the [correctly sized for the wire] building OCPD will trip and protect the premise wiring; which is the whole point of the OCPD.