Thanks for your comments, Al. I won't shill for pageviews, so here's my response there:
I reckon my perception of the difference comes from having them side to side, and in listening my preference seemed pretty clear to me. Don't think the measurements are telling the whole story...don't think they ever do, TBH. They both have their merits, and I do like the more refined sound of the 650. I think with proper care and feeding (see your comments on cables and balanced/really good amps, which I agree with) the 650 might surpass the 600 for me, but I haven't really sussed that out.
I agree, both these cans play well above their price in today's market.