Sounds like ya'll are having entirely too much fun with this process.
Some other general points I might add to Cam's list:
- Hot -- leave it on for a few hours. If the machine is hot through-and-through, it'll maintain temperature better. Optimally, the shot should pull at 95.0C (give or take, depending on roast). Anything different will result in either sour or bitter espresso. I leave my machine on overnight, and use a Fluke with thermocouple set underneath the grouphead to measure temperature.
- Fresh beans -- they really do make a difference. Beans are at their absolute best between 3 and 7 days from time of roast, and it takes a lot of effort to get anything drinkable out of beans more than about 30 days old. If you produce a lot of espresso, it may be worthwhile to learn how to roast your own -- it isn't anywhere near as complicated or expensive as you may think. Google "dog bowl heat gun coffee roasting," and you're home free. Seriously. You might also try using a pound from ccmcoffee.com, or intelligentsiacoffee.com (Black Cat is amazing!) just to see what professional roasters do with coffee. If you can't tell the difference, then don't bother with that experiment again.
- Pressure -- 9bar really is optimal, though if your temperature isn't spot-on, the flavor differences there will overwhelm any pressure problems you may have. Point being: pressure is, in your case, a good indication of tamp pressure, but it shouldn't have substantial effect on flavor.
Rock on, guys.