The reason we use felt on our lesser vintage orthos is because their resonant frequency is in the bass, and the bass is where the diaphragm has to really move. That's lucky for us, because we can just add an acoustic series resistance and preferentially (though some might say differentially) suck the energy out of the resonant peak. But when the diaphragm isn't displacing much air, as in the upper midrange, damping won't help. That's why with the Staxen and A-Ts and in the single case of the YH-1k, the designers resorted to absorptive pads, and often this wasn't enough, as anyone who's listened to an un-EQ'd TK-33 knows.
What about the T50 & the OEM versions like NAD RP18?
They seem to apply a similar absorptive yellow fibereglass pad.