I agree that Grado is doing different, but not necessarily better. Ditto Sennheiser.
I think the DT880 is Beyer's best headphone yet, so I think they're still improving.
I also think Audio Technica's L3000 is their best (dynamic) headphone yet, so it seems that they're more or less still improving as well.
AKG made a bad decision getting rid of the K1000 -- that is such a different headphone, it's kind of hard to beat. I'd love to see them do a flagship version of that. That said, they've always made good headphones, so they're kind of hard to criticize, of course they're going to stumble, eventually.
Long and short: unlike the (headphone) amplifier industry, which is definitely improving, the headphone industry, while not necessarily on the decline, still has room for improvement as well. It's just like stocks, sometimes they're up, sometimes they're down. I think we're in a "noisy" area of the graph, where the overall trend has plateaued (?sp). You know, as much as I hate the PS-1's inordinate amount of bass, I can't criticize the quality of the bass -- perhaps it would respond well to EQ?