I auditioned the Sennheiser HD280 PRO and I own the Grado SR-60 cans. I feel that both headphones need an extremely high number of hours to burn in before they blossom in sound quality. These two headphones can not be more different from each other in terms of design and price. The HD280 PRO is a closed circumaural pair of cans that have a relatively high level of passive sound isolation and an extremely neutral frequency response while the SR-60s are open-air circumaural cans with a legendary history for warm, harmonically rich, and smooth frequency response. In my humble opinion, both cans represent excellent values for mid-priced full-sized cans. So, you really can not go wrong with either if you decide they are worthy upgrades.
If I were you, then I would audition these two cans before investing your money. If that is not possible, then continue doing research into both cans as they have a storied past within the purview of Head-Fi. My gut tells me that the Grado SR-60 would represent a better price : performance ratio of the two aforementioned cans. There is no doubt in my mind that they will give you added resolution, tighter bass response, and a slightly fuller soundstage as compared to your Sennheiser PX100 cans.
No, I won't sell my Grado SR-60 cans, but I would recommend them to you especially if you are looking for an alternative to the Sennheiser house sound.
As an addendum to what I have written, remember that you can decide to re-encode to a higher sample bit rate or utilize a different audio compression codec from the source material. In doing so, you will achieve greater transparency which may be a boon should you decide to get either the HD280 PRO or SR-60.