Well, what a trip! I sure have learned a lot in the past few days, aided immeasurably by the great folks in this forum. (My wife says that at least I'm providing entertainment to the members of this site! Not sure what she means by this...)
In any case, I think I finally understand what is going on and am making progress with consistency. Getting a reasonably accurate scale made all the difference. I now can dose and distribute by volume and achieve remarkably accurate (+-0.1g) weights in the basket.
19.3g ground weight == a full 58mm basket with Cherry Bomb blend from Dark City Roasting in Toronto
16.1g ground weight == a full 58mm basket with indigo Blue (same supplier)
I have been repeating this dosing over a dozen shots and if I was so inclined, could probably eliminate the weighing altogether and be confident of my consistency. Since it is really very little trouble to weigh each basket I will continue for the time being.
I also am now getting very good looking pucks so I think my tamping is good and is consistent. By good looking pucks I mean that they are intact, show no obvious signs of gross channeling, reasonably dry and when I break them apart in various spots they seem to have the same amount of moisture throughout the depth. The next step would be to get a naked portafilter, but since I'll be changing machines within a month or so, I think I'll wait for the new machine to buy it.
My pours are excellent in terms of crema: almost all crema as it extracts, settling out to 3/8" to 1/2" thick. So I think I finally am getting close. But 2 issues still remain: extraction time and taste. (The beans I am using are within 4 days of roasting.)
Taste:
The Cherry Bomb blend is consistently lemony. My guess is now that this is due too low of a group temperature.
The Indigo Blue blend is much better, but I think could be better if I could eliminate a slight bitterness. Since i just started experimenting with this blend I have not reached a conclusion.
Extraction time: note that currently I'm holding the espresso volume constant at 2.0 oz.
The Cherry Bomb blend at 19.1g consistently extracts too fast: 18-20s, even with finer grinding. this morning I stopped adjusting finer and moved to the Indigo Blue.
The Indigo Blue blend at 16.1g extracts even faster: 10-12s! I have only pulled 5 shots with this blend, each time grinding finer. (I needed to stop before my head exploded and my craving for a morning coffee would have eliminated my attempts at scientific objectivity!) Even with a 12s extraction, the espresso was very worthy of drinking, especially in a morning cap.
So, where to go next? I think I need to get the extraction times under control. My inclination is to leave the dosing alone since I finally have consistency and keep moving the grind finer and finer until I get better extraction times. Comments please, and once again thanks for everyone's help.
Martin