Hello, I have recently just bought a replacement pair of ear pads for my m50x's (these).
so far comfort wise they are a godsend, the original ear pads on the m50x's are just a tad smaller than my ears, so i don't get a complete seal and it makes them painful to wear for extended use (I use these to game and listen to music). these are slightly bigger in width and height and massively lager in depth, letting them fully cover my ears.
 
The only problem, there is a definite loss in the bass factor, losing the thump and pound of the bass, that I really enjoyed and made some songs come alive for me.
This could help solving the problem but im no expert on headphones.
With the new ear pads they have no holes in the inside of pad itself (check by turning the ear pads inside out)
And the wall of the pad is a lot thicker and that could be covering the vent holes on the plastic housing of the driver preventing proper circulation (the row of holes top and bottom of this picture)
 
Any suggestions of what i can do to these pads to improve/solve this issue or any other pads i could try?

If you got the velour version of those HM5 pads, that's where you went wrong. Get the original size (non-memory cushion) pleather HM5 pads and you will be back in bass heaven. The comfort of them is better than the memory foam version, IMO. They're much softer and not quite as deep.