Super image Olly and the bluntest tools can produce delicate results in the right hands which you have proved here.
I've delayed responding as I wanted to check my own data. The subs I have at 860mm don't show the shock wave as it's just out of the frame but it is visible pretty clearly in the version I took with my FSQ85 and I didn't remember having to work too hard to drag it out... but going back to the original stack, it's pretty faint and gets a bit lost in the OIII signal that is pretty widespread across the field of view. Here's a crudely stretched, and very noisy crop:
I wonder if you got a similar result? If not, and if you did your trick of holding down the background noise when stretching the data, could that have suppressed this feature?
As I say, lovely image...
Ian