btw, a tech, in many cases has little experience being a barista. take their advice on possible portafilter side issues with a grain of salt.

Truth. BUT I think many of us are expecting both the OP and the folks that have been in to work on it would have suggested at the very least swapping the screen from one side to the other. Don't forget, LM has done some troubleshooting on this one, and in my experience, they're pretty darn good at suggesting many of the things we've suggested here.

La Marzocco GS3 non vertical flow fix

The above thread has me wondering if perhaps this was the same issue that was causing the uneven extractions with the Strada from this thread. That I'm aware of, there was never a solution to the OP's extraction problems and the thread went cold...

Any thoughts?

Would have like to see a video of the water flow exiting the screen and then one with the screen removed, to see if the water is dispersed evenly.

Capitol wrote:La Marzocco GS3 non vertical flow fix

The above thread has me wondering if perhaps this was the same issue that was causing the uneven extractions with the Strada from this thread. That I'm aware of, there was never a solution to the OP's extraction problems and the thread went cold...

Any thoughts?

Unlikely, on the MP the valve body is esentially a large nut thats mounted flush on the group head, if its not flush it would leak and if its tilted or cross threaded then the shaft will also have a slight tilt and wouldnt go through the group cap.
The AV/EE uses a group tube so it is quite possibly thats tilted.

http://lamarzoccousa.com/docs/tb/TB47-M ... dParts.pdf

i know you said you replaced the burrs but it could still be the grinder. (non parallel grinding plane)

is there another, possibly better grinder that you can try?