I heard the 300B today. It is really, really special. It is the best headphone amp I have heard to date.
It is very musical. It is not lush. It has great PRaT, yet does not sound bright. It has amazing transparency that brings the musicians to you that you at times forget that you are listening to gear. Apparently it was originally designed only for headphones as Pro gear, and it delivers.
I briefly compared it to the Stingray. Stingray sounds veiled and flat in comparison and does not extend as well along the musical spectrum. The 300B is richer and more musical. It has much more resolution, but the detail is presented as part of the organic musical whole instead of thrown as pebbles at you by the analytical sounding gear. The good news is that the Stingray has a similar sonic signature to the 300B as both have a lot of PRaT.
If there is anything better than the 300B, I would love to hear it.