I read about somebody on here doing that once, though on a 580 rather than a 600. I wish I could find the thread. From what I gather, the Sennheiser parts department doesn't really care what you buy--theoretically if you knew all the part numbers you could build an entire 600 or 650, albeit likely at significantly increased cost over just buying one outright. Plus, you don't get driver matching, though I wonder how much that really matters these days, since these headphones are both such mature designs. Sennheisers tend to have impressively small unit to unit variation in measurements, so I imagine you'd stand a pretty good chance of getting a good match just through two randomly selected drivers.