I'm also struck by the range in budget for otherwise well-matched upstream electronics for the HD-650. Just within the Schiit line, you can start at $200 for Modi/Magni, and move all the way to $1,600 for Gungnir/Mjolnir (plus a $300 XLR cable), and find little but praise for HD-650 pairings with either rig. And Sennheiser itself expects to sell new amplifiers for $1,600 and $2,000, quite reasonably expected to pair well with the 650, even though the 800 may be the better match.
So that rather complicates advice on how to allocate a value-optimized (definitely not cheap) head-fi budget (putting aside the obvious question, "does the high end rig sound 8x better than the low end?"). Although, if you bundle the XLR cable into the headphone cost, the HD-650 ranges from about one third to two thirds of the HP/amp/DAC budget within the Schiit line, which seems like a plausible recommendation. Of course the over-the-counter XLR cable nearly doubles the cost of the HD-650, and adding the XLR cable to the electronics category drops the headphone allocation to 20% for the high-end option, which starts to beg more headphone spend.
Conventional wisdom is to allocate a greater proportion of the budget to the headphones, so $2,000 of upstream electronics would seem to stretch that advice for the ~$400 HD-650, not to mention going with even more expensive DAC/amp options. Of course this assumes only one set of headphones, so adding additional headphones to the mix or dual-purposing the DAC for loudspeaker use would bring the budget allocation into line. Not to mention that upgrades are lumpy; owning the HD-650 and stepping up to a $1,600 DAC/amp sets the table to make a subsequent step to the HD-800, which invites another DAC/amp upgrade...