My experience exactly, though I am only half way there... I switched to XL pads and fell in love with the improved comfort, but the reality for me is that the sound changed just as you described. I have been spreading the love a bit for the M-100 because I think it is a great value for those wanting good sound at a fair price with some decent portability. If I hadn't of ripped the stock pads trying to get them off, I would be back with them right now for exactly the same reasons you described above.
WIth the XL pads, they sound very nice at very low volume, like a "loudness" button on an old stereo receiver, but crank them up and the bass craps all over everything. Not as bad as other bass head phones, but bad enough for me to lose interest.
I am just going to live with the XL for little while longer and order some stock pads soon. What a bummer. If only the stock version of these cans were a tad lighter, had cups that could rotate just a bit more from front to back when on your head (they don't do this at all, and I feel pressure more at the back of my ears than the front because of it), and had just a tiny bit more comfort in the stock pad.... they would be perfect. I think the stock sound is "on the money" and that is why I will still be keeping these for a long time.