Can't see it in the screenshot, but given the hardware bus is exactly 6dB higher than the Master bus, I would guess there's a parallel Send on the Master bus to the hardware out.

Well I think I've got the source of my problem, after ensuring that no tracks or busses are bypassing the Master Send.
 
Years ago when I had Sonar 6, I had created a template as a basis for each new project.
Then years later I went to Sonar X1 producer and loaded that old template, tweaked it a bit, and re-saved it.
Then a few more years later, I grabbed that template and brought it in to Sonar X3, tweaked it a bit and re-saved it yet again.
 
So I had a thought to delete the Master Bus, and add a new one back in.
Wallah! things went as expected,
 
So I've scrapped my dodgy template and re-created one from scratch.
All looks good now.
 
Thanks so much for all your helpful advice.
Adrian
 

I've seen a lot of issues migrating templates & projects across multiple versions of Sonar. My latest is similar to this one... but different.