Yesterday I replaced the pads on my T51p. My set is a very early one, and the pads were becoming rather flat.
The new pads are quite different: whereas the old ones are open at the back, and are glued both at the periphery and around the centre of the baffle ring, the new ones are sealed, attaching around the edge of the cup with a self-adhesive ring. They are made of a softer material and isolate rather better.
The sound has also changed. There is now a soundstage worth speaking of, and the balance is more neutral. The price is a slight loss of immediacy (presumably as the drivers are now further from the ears).
On the whole it is worth doing. There remains a slight resonance, audible on well-recorded solo piano. Presumably this is due to the sub-optimal cup volume. At the end of the day, “there ain’t no substitute for cubic inches”...