OK, next question on this....
The whole reason for having this Roadtech one, when I already have a normal Garmin Zumo 590 (I've installed 595 firmware for the extra features) is so that it will interrupt whatever part of the boom audio I've got playing with Route instructions - only the RT model does that.
Now as this is a dealer demo model, I want to verify that works! I got it to load a map of sorts yesterday, put it on my bike, selected another audio source, calculated a route, and waited. No interruption.

(This was done static in my garage, bike is now off the road for winter) Bike does have the Audio Jack Jumper installed, and the Boom audio flashed at the dealer so I can hear audio from the normal Garmin Zumo). Switching to the Aux input (the Garmin) and calculating a route again, Digital Doris does give an initial instruction - "Please drive to Highlighted Route" - but that doesn't interrupt other audio sources.
So, two questions for those with the RT 590 who use it as described as above:
1) Is the tone it sends telling the Boom Audio to interrupt whatever source you are listening to audible?
2) Would you expect that initial instruction above to interrupt other sources?