I have taken the step of buying a second DSW from a fellow HH. (It was a pleasure doing business with you Darren).
The sub is installed. Daisy chained to the first one as LFE channel.
Calibration set at -5.5.
Tried Passengers scene which was causing problems, volume to 75. Not a problem.
Went up to 80, and it was completely fine. Shook the room but no issues with subs.
Looks like we have a fix.
Consider it an upgrade as well.
Good luck to everyone who has suffered the same issues.

Cheers and Happy New Year to you all

Nick

861v8+ID41, HD621, DSP6000, DSP5500HC, DSP6000, 2x DSW's, Roon, AudioQuest Niagara 5000, IsoTek Titan, Nova, Vision, Mira.