I have done my research and have settled upon the Bottlehead Crack for the HD650. But once I started thinking about it, I realized I would only upgrade the HD650s to something else, something better, most likely an LCD-2 or LCD-3. And with this new mindset, I set out to find an amp that would be able to make both headphones sing. In my search, I found the new ALO Audio Pan Am, which, at $600 is just a bit more than I'd like to spend, but definitely doable. Multiple reviews, including Michael Mercer love the Pan Am with the LCD-2s and 3s, as well as the HD650. So are there any other amps out there that you guys know of that really can drive them to their full potential? I would say my absolute max budget is $700, no more.