Regarding the slight LF bump and isolation, I think you're spot-on with the 280's, acidjack. They're great headphones but the M50's sound just a shade better, to my ears at least. One drawback I will point out about the Sennheisers, though, is that I think they're built for "narrower" heads. If your melon tends to be on the wider side, they can be uncomfortable.
Congrats on the new purchase, Tablet! They'll only get better as you burn them in and use them more.