I'm planning on getting the HD650 and Micro Amp in a couple weeks for all the reasons you mentioned. I've exchanged several emails with Jorge at HeadRoom and he's very responsive and helpful and his service alone has pretty much sold me on trying any of their products. I'm very happy with the carrying case/bag I ordered from them earlier this year. I'm also excited to try the crossfeed as I listen to some old jazz recordings that I bet could benefit.
From my experience I'm a fan of headroom. I recently upgraded from my fiio e7/e9 to a headroom desktop amp + power supply for roughly a grand. Was it worth the upgrade? With the hd650s, you betcha. With my pro 900s though I can barely hear a difference.
Overall their products are a little expensive, but with that expense you get fantastic build quality, unbelievable support, a 30 day trial money back guarantee, and that warm fuzzy feeling of supporting an American business that's just doing things right. I had actually originally ordered the micor dac & amp combo but decided to cancel it and go for the desktop.