Oh, my question was purely on the headphone output side of things. Since the WA uses a phase splitter internally when being fed single-ended, all amplification paths are fully engaged even when connected via RCA, and the resulting output levels of the respective headphone outs are the same in both cases. (I definitely like this as an architectural design decision).
Speaking of SE vs balanced input, I want to mention the following -- even though it may call into question my own listening preferences (haha)...
My DAC outputs balanced and SE simultaneously and I was trying to decide which of my two amps would take the balanced connection and which would take the RCA connection. So as part of that test, I had the DAC (Metrum Pavane Level 3) feeding the WA33 using balanced and SE at the same time, and was able to switch inputs coming from that same source instantaneously. And the headphones (Susvara and Auteur) were being connected through XLR.
I actually liked the resulting presentation as much (and maybe even more?!) coming out of the SE compared to balanced. Balanced frequency response extended ever so slightly deeper and I think it was just ever so slightly more punchy, but SE resulted in larger (arguably less precise) images in a slightly expanded soundstage, which I quite liked. I also think the mids were less forward. But all the differences were -- as you might expect -- quite subtle (and c'mon, for an amp of this level, they better be).
My only point really is that RCA is a viable way to feed the amp and IMO doesn't have to be thought of as a second-class connection. And is just generally worth trying out even when you have the option of using balanced.