The coffee mattered, but only a little.
The tamper mattered, but only a little. I swapped out my flat 53.0 mm Reg Barber for a 53.2 mm convex. A bit of improvement. I purchased a very nice Thor tamper (thanks Les!) and that was only a bit of improvement.
I installed the Programmable Preinfusion about 3 weeks ago (those side panels suck!).
After a week or so of fiddling around, it is now so simple.
- 2 blank shots
grind and slightly overfill portafilter (Mazzer Mini)
level with a knife (no tapping)
tamp (either tamper) with a slight twist
tamp again a little deeper. Now a few millimeters below the internal ridge.
Pull the shot. Around 1 1/2 ounces or so in 25 seconds.
ZERO Channelling. Coffee expands to the ridgeline, slight indentation from bolt.
A little soupy water on the surface. Soaks in while checking. Did I tell you no channelling?
I am finally happy with my Espresso Machine (until the next issue arises and I obsess again, but that is my destiny). Thanks to all of you for the advice I have found on this forum. You are making a big difference!