Andy .... You stated that you tried out the AR500 with an XXL Controller....did you happen to notice if the XXL Status LED flashed indicating a lost signal when you turned off the transmitter?

The Scorpion XL indicates the status of the R/C signal by various blink patterns on the
Status LED. Here are the blink codes:
Solid - all signals good, motors enabled
Slow blink - searching for required radio channels especially at startup
Fast blink - R/C signal lost or pulses out of legal range
Very fast blink - Calibrating radio signals (after Cal jumper placed see below)