Everything affects channeling. You have to check every link in the chain. That said, the culprit in my rig turned out to be the VST baskets. My Bezzera likes to brew at 12-13 bar. The VST, in my opinion, is not optimized for that kind of pressure. I switched to the EPHQ baskets. Problem solved. Of course, all else is perfect (distribution, grind, level tamp, etc).

I agree with others...the hardness of the tamp doesn't make that much of a difference. Once you press all the air out, further pressure makes no difference.