Hey, Guido...
Yea, I know. The "futtering" you describe is perfect for the sound I get on poorly aligned cones - but only when slightly mis-aligned. It can be very subtle, but needs be gone.
I wonder if covering the "vent" is merely reducing the cone movement, and reducing the "fluttering"... ?
Still "sounds" to me like one of those reconed K140's ain't quite right. And anyway, if the K140's are same vintage, and a pair - shouldn't they be the same?
If you can hear it, and it's mechanical (and not sound...), I'd say it ain't right.
Oh well...