I think a crossfeed sounds worthwhile, which is a primary motivation for balanced output, but it's a really huge step economically. Putting aside DIY cables, XLR cables for the HD-650 that I've found are $275–$300. That's a big expense for a $350–$500 set of headphones. A Mjolnir amp will set you back $750, never mind a DAC. That puts the headphone proportion of the investment below 33% (approaching 25% with a decent DAC in the mix), which seems awfully low, given that even Schiit will tell you to allocate most of your head-fi investment to transducers. Put another way, you'd have to quadruple your investment in the 650 to adopt balanced output.
Now, with the HD-800, the proportion comes back into line, with about 50% of spend on headphones, including a DAC. So if you have your eye on eventually upgrading from the 650 to the 800 (or similarly-priced 'phones), it could make sense to use the 650s with balanced output in the meantime, as I'm sure they'll deliver the benefit of the crossfeed. Still, using the 650 paired with a Dragonfly or Modi/Magni stack as a baseline, you'd have to quadruple the investment to arrive at HD-800s with balanced kit upstream, an outlay of more than two grand. On the bright side, for three grand, you'd have a an awfully high-performing head-fi rig, leaving little on the table before moving into cost-no-object territory.
Um, Crossfeed? and yes, my plan is to get the hd800's (or the like) in the future and run them balanced... most likely with the mjolnir
Thanks for the input by the way.