Got the HSA1b earlier today. You nailed it.
Fresh out of the box and in comparison to the JotR, the RAAL amp projects a larger soundstage in all directions but especially forward (depth). The congestion I complained about earlier in this thread is gone. The separation, as well as layering and imaging is excellent now. I feel the amp also resolves more microdetail such as room reverb, air around instruments and the textures of sounds.
In terms of tone, the sound is darker and slightly warmer as expected (pushing the SR1a more towards neutral rather than lean and bright). Beyond this, the overall sound is more naturally liquid, without resorting to a smearing of transients (like the Black Dragon cable does, however subtly). Overall, this makes the HPs more forgiving of recordings and I don't find the SR1a as harsh as they previously tended to be. Interestingly, It's also easier to notice changes in recording tone and space, just without the edge now.
The improvements are significant and readily audible but just like you said, it's that extra 15% audiophiles yearn for (though these estimates are wildly subjective). I never expected it to be night and day, and it isn't.