I have both flat and curved face tampers and have tried to determine which works best for consistent shots. I don't see much difference between the two, however when I tamp first with the flat face tamper and then follow up with the curved face tamper my shots have improved. I just use a straight, single tamp with each tamper and no polishing. I suspect that the curved face tamp causes the puck to expand laterally and lock into the basket better. At any rate, this is my standard procedure now and works great for me.

Interesting twist on tamping, but I am skeptical because (a) in my experience, tamping is low on the important contributors scale, and (b) multiple tamps risk breaking the puck/basket adhesion, leading to channeling. That said, to test your theory that double tamping improves your espresso, try the technique some of the reviewers used during the Titan Grinder Project:

• Remove the basket retainer clip from the portafilter
• Source two identical baskets
• Discreetly mark one basket under the lip
• Prepare both baskets, one using your multiple tamp method, the other with a single tamp
• Ask an assistant to pick one of the baskets without your knowing which is which (or randomize them yourself by spinning a Lazy Susan)
• Pull the shot, take taste notes. Repeat for the next shot.

If you can identify the multiple tamp shot four times in a row, you're done; otherwise see A note on comparison tests for how many trials are required to prove significance.

It is good if to add a visual estimation of a condition of the puck. Or the channeling only because of a distribution error?

Rostik_KIEV wrote:It is good if to add a visual estimation of a condition of the puck.

I don't bother looking at the puck, other than to confirm it's consistent shot-to-shot. Any fractures you might observe and attribute to channeling are more likely to have been created by the 11 to 0 bar depressurization in less than a second than the leisurely pressure build up during the extraction.

With practice, it's a lot easier to taste the effects of channeling than see them. To sharpen your taste acumen, intentionally pull a bad shot and see how it impacts the flavor profile. Visual evidence of channeling via a bottomless portafilter only reveals gross errors in barista technique.