In spite of my green enthusiam I certainly wouldn't doubt that something like the hd800 has much better sound, not that I've ever heard them.
For me reference class is live acoustic music, especially including classical or other symponic/orchestral genres in a good hall, minus the full body bass experience that you could never hope to get with headphones. The DT660 does not match my reference class and I would certainly hope an open headphone costing ten times as much would come closer.
In some ways though it's hard to say "not in the same league". Again, I would hope the sound quality is not in the same league, but then if you make a big rig tractor trailer that can run at 165 mph, I don't think it's fair to say that it's not in the same league as a Ferrari Enzo. I mean ok, yes in sports terminology it would race litteraly in a different league, but the implication of the phrase is that it's a lower league, where actually I'd be more impressed with the truck.
It might well still be true that the dt660 does not deserve the same category of respect, but it is a different thing, and it does its thing well, certainly for the price, and probably even for a much higher price.