By chance I located a KMC XL10 chain that I had purchased long ago, and replaced the chain with it.
There was definitely some stretch in the old chain, visible as I lined it up with the new one.
I also played around a little with the B-tension screw, moving the derailleur up and down relative to the sprockets. However no position proved to lessen the vibration. Perhaps now less audible, it is still felt in the pedals.
So.. new cassette, pulley wheels, chain. Only thing not replaced in the drivetrain now are the chainrings on the crankset. However when I turn the pedals, the frequency of the vibration indicates that this is a problem with the rear part of the drivetrain, unrelated to the chainrings.
Note that the crankset inc. BB is recently dismantled, cleaned, lubed and mounted together according to manuf. recommended torque. Bearings rotate smoothly.
Problem remains unsolved.. :-)