The sax is wonky with it's power implementation; the overload point varies by the voltage you feed it, but the output doesn't. However, I'd had mine die during a show due to lack of power (from a regulated supply) and it just dies, it doesn't fade out like this. The tekkeon is a regulated power supply, so you shouldn't see a sag in voltage like normal batteries, it's basically all or nothing.
I'm not sure I have an idea of what caused this...
edit: missing a phrase (now in italics).