Update:
I posted it in another thread, but I will also post it in this one.
My Beyerdynamic DT880/600's arrived from B&H Photo yesterday.
I also received a pleasant surprise from Bill Me Later, in an email yesterday, that said B&H was deducting $20.00 from the original charge! So instead of paying almost $305.00, I am only going to have to pay about $285.00.
I tested them, and they are indeed the 600 ohm version. I tested them with a multimeter, and one channel measured 597 ohms, and the other one measured 605 ohms.
I haven't really listened to them, as I have them breaking in with pink noise right now.
I can say that my older version Headroom Micro Amp can drive them to very high levels, actually uncomfortable levels when set on high gain. I do have to turn the volume control about 1/4 higher on them to get the same levels as my Beyerdynamic DT880/2003/250 ohm. I tried 12:00 or 12:30 on the volume knob on the 880/600, and it takes about 10:00 on the 880/2003 to get the same levels, all this is with the high gain setting on the Headroom Micro.
From brief impressions, the 2003's sound more extended in the top end, and on very bottom compared to the DT880/600.
The 2003's also sound slightly more open and alive to my ears than the 880/600's.
The mids are more recessed on the 2003, and more forward and filled-in on the 880/600.
The 880/600's do sound better balanced overall, but...
Right now, I slightly prefer the sound of my 2003's, believe it or not, but that may change after the 880/600's break-in.
I will post more after they fully break-in.