HD5x5 pads are the same, just velour instead of pleather. I swapped my 380's pads with 598's, I thought that they both sounded like crap afterwards. You should try it out if you are curious, lol, just don't hope for anything good. They could each be made to sound good with the different pads AND the right modifications, possibly.
I read some guys discussing aftermarket upgrade pads for 380 from some UK-based seller on the bay. They are pleathers as well.