Personally if you're forking out for headphones and DAC/HPA then I would opt for a balanced (XLR) configuration. As your headphones are close to your ear noise matters. A true balanced output offers noise cancellation, the shielding of the cable isn't the return path for both signals helping reduce noise and increase channel separation.
Balanced headphones simply means the headphones bring the two wires from the speakers to the headphone socket (ie has 4 connectors on the plug). They can be used on normal amps by simply using an adaptor that connects the left and right grounds together. (Note you may be able to use a balanced amp with an unbalanced headphone but this depends on the amp/dac).
DSD512 digital to analogue conversion itself is very simple, there's no expensive low tolerance resistor ladders etc. Additionally most seem to be based on the same IC chips leaving power supply and PCB layout being the only hardware differentiator. Accuracy of clocking is important as DSD512 data streams at 24MHz this also means a good DSD streaming device such as a PC running a DSD capable streaming software. A fast USB connection would be paramount - USB3.0 etc.
Transcoding from PCM->DSD (ie SPDIF->DSD) means degradation of sound.
I'm designing/building a balanced HPA with DSD capable DAC. I was thinking of 660S or Grado for balanced headphones. However given the cost I would audition the headphones as they each have their own sounds.
Topping is one of best regarded, you could look at Audio Science Review. They get a little audio-nuts on the details but they do provide some semblance of objective measurements using professional audio test gear. However nothing like listening yourself.
