i have used E.A.T. dampers on tubes in a cd player. they have been in production for several years. they are aluminum on the ouside, with several teflon inserts placed in the inner grooves.
at first i thought they "warmed" up the sound, i.e., reduced harshness and smoothed out the sound. now, i am not sure. i recently compared a damped and undamped tube and did not hear much of a difference.
as for herbie's tube dampers, i thought they increased resolution and reduced warmth. i do like herbie's other products, such as anti-resonant devices.