I was merely replying to the title of the thread, which focuses on a truly balanced phono stage. To me, it would be a waste if you had a phono stage like the XR-10B or the Boulder or the Pass Labs, to use it only for headphones.
It would have more of a WAF if you also had it hooked up to a nice entry level high-end speaker system.
I have had the opportunity to compare the RSA to the $29,000 Boulder and I could not hear a difference. Same as the $7500 Manley Steelhead.
If you are fortunate enough to own a VPI HR-X or a pair of Watt Puppies, or Utopias, then the XR-10B is well worth it's asking price. Again, the question posed was about true balanced phono stages. If you have a Pro-ject or even a Scoutmaster, then you should obviously look elsewhere. It's all relative.
What would you use for this Continuum Audio Labs Caliburn turntable & Cobra tonearm (it's a mere $89,900):


...or, you can step down $20K and "settle" for this Rockport Technologies System III Sirius turntable and tonearm:

Dreaming is a huge part of any hobby, including audio. There's more to life than a $300 portable amp and asking
"What's the best headphones for under $100" 