Pre-infusion 6.
Shot pulling at 10.

I actually don't love that; 9 preferred...

Oh, and the group does drip drip drip whist idle.

I defer to the LM experts but I would have said 85psi line pressure is way too high for feeding an espresso machine.

Yeah, me too!
He explained to me in great detail though why it was just fine.

The brew pressure is now 10bar though, strangely, so I'll open her up (again), and adjust that down to 9 today.

Hoping to get everything dialed in just right over the next couple days. I'll add to this thread if any problems manifest.

Update:

I think I'm finally dialed-in, and I hope the following comment can help others in the future:

The "buzzing" of the needle on the pressure reducer seems to have been a function of the way the motor was drawing, and not a problem with pressure or flow. I made sure that pressure (85psi) and flow (filled a litre in about 2 seconds when free flowing from line) were fine. Although La Marzocco said 85psi is perfectly safe for the machine, I was still unhappy because that means 6 bar during pre-infusion, which is not exactly "pre" infusion. So, I moved the pressure regulator to under the sink (ie, before the filter), and set it to 50psi. Everything is lovely now. No needle buzzing when brewing a shot, and the pressure hangs steady 45/50psi while brewing.

Thank you for taking the time to update.

It is great to see issues like this come full circle.

Cheers!

- Jake