I know I'm the weak link here since I was using someone's amazing setup: La Marzocco GS3, Mini-Mazzer, Stumptown beans ... this should add up to espresso greatness but I'm killing shots (and not in a good way). Any troubleshooting help would be appreciated ...

The main issue is this - if I fill the portafilter to the brim, distribute evenly and carefully - everything pre-tamp just like would normally - when I go to pull the shot I get no flow and the GS3 stops after 10 seconds due to no flow. A few very dark drops appear on the bottom of the bottomless pf, but no flow like I'm used to seeing. I can dial back the amount of grinds so it only fills half the basket and get a mediocre shot (moderate striping, decent flavor), but my distribution is compromised since I can't get a nice flush surface before I tamp. I'm not changing tamp pressure, grind, temperatures - anything else ... on every other machine I've been able to start with a distributed dose that's flush with the top of the pf pre-tamp.

I don't want to live with uneven distribution and mediocre shots when I use a setup this beautiful ... thoughts?

Down-dosing: how to level when the basket isn't overflowing? offers technique suggestions, but I'm curious... what basket are you using at how much are you dosing? The La Marzocco wasp-waist baskets require around 17-18 grams to reach the point where a 58mm tamper won't bind. If you want to dose less, try using to a 57mm tamper or switching to a straight(er)-sided Faema-style basket will help.

If you want to reduce distribution to rote mechanics, the WDT and Stockfleths Move for Dummies (or nutation) are nearly idiot-proof, albeit a bit OCD.

Someone else's machine, not changing grind, an unfamiliar grinder, using the methods that worked on another machine... did I miss anything?

You filled, leveled, and no flow... So grind coarser.
Distribution is not necessarily correct by leveling off. The top surface of the coffee can look smooth and even when there are voids beneath. Check into the two distribution techniques Dan mentioned.

Also, try this: Wipe the screen dry, then dose, level off, tamp, lock the PF, then remove it and see if the coffee is hitting the screen. If not, then just grind more coarse as I mentioned above. If the screen is impacting the coffee then you will have to use less coffee.

Haha, Randy, point taken! Yes, the change in setup should have been enough to anticipate issues, but I was too blinded by the beauty to take that step back and realize it.

To answer some questions: I was using a version of the WDT since it was too low for my usual Modified Stockfleths. The basket is a ridgeless, smooth sided basket. I did adjust the grind somewhat and was able to get significant improvement after the initial post ... Once I adjusted the grind it dropped from about 22g down to 18g per basket with a nice smooth flow. I thought the grinder had already been adjusted to the Stumptown Hairbender but clearly it hadn't (not sure what they had been using before). As a side note, once I got things tweaked, since I was using the bottomless PF for the first time I was able to see that my usual technique did give me a little more side channeling than I would have expected so I added some nutation to my tamp with beautiful results. And, more importantly, delicious results.

Thanks for the input!