Yes, rock solid.
Update: The nylon washers tamed the vibration a lot, they could only go on the top of the board as there was a grounding pad on one of of the stand-offs. Still a slight noise, but much more ignorable. Huzzah.
However, I've decided that the amp needs a decent service. It's working and sounding great, but by the looks of it it's not been touched in 25 years, so I'm dropping it off for a thorough check-up thus week - something that my maintenance skills aren't really up to.
It's Trace Elliot fun for a short while, whilst it's having it's holiday
Thanks for the help folks !