FWIW...IME...My 2 cents...etc...
I've only been at this for 4 months so I'm still a novice and could definitely be wrong. The sound engineers among us can correct any of my misperceptions and errors. That said, this is what I think about these various materials:
Paxmate Plus, Silverstone, and AcoustiPack Lite are open-cell materials used for acoustically dampening the cups and baffles. They absorb unpleasant reflections and reduce standing waves. AcoustiPack Lite is the only one of the three that has a constrained layer, so it is also useful for mechanically dampening cup and baffle vibration/resonance from the drivers. Adding fiberfill, cotton, wool, etc., loosely placed inside the cup actually increases the perceived size of the cups and may improve soundstage and airiness. Reflex dots of tape, plastic, felt, rubber, etc. may be added to the back side of the drivers if there is a need to boost euphonic treble reflections to your ears and seem to reduce bass and/or tighten the bass.
Dynamat, FatMat, and B-Quiet are butyl-based materials with an aluminum (?) constrained layer used for mechanical dampening. They are designed to convert mechanical vibrations into heat, thereby removing an unwanted source of resonance that would otherwise mess with the SQ. They are intended for application on the external surface, surrounding the drivers, not inside the cups. Nevertheless, some here on headfi have reported good results with installing these materials inside the cups and even on the outside of the cups. If you decide to use these products on the ear side of the baffles as I have done, you must remove the ear side dust cover felt from around the drivers, down to bare plastic. This is the only way these products will work as designed. You also should cover the constrained layer with self-adhesive felt to nullify reflections from the aluminum.
Both products help to clean up the sound quality by removing resonances and reflections from the stock T50RP's so their sound quality goodness can be heard. I find the combination of Paxmate, cotton, tungsten putty (or plasticine), and Dynamat work well together, in the proper configuration, by tightening and extending the bass, clarifying the midrange, and extending the treble.
Paxmate, or similar, placed in the cup outer ring provides a better seal with the baffle and may further reduce mechanical resonances.
As always, YMMV.