With the lyrics kerfuffle root-caused, I don't have any other functional issues with the Walkman. It's become my main playback device these days. I'll plug in my regular headphones into it directly, and if I want to listen to my electrostatics, I'll use the USB connection to use my Ferrum Erco or I'll plug the Erco's RCA input into the 3.5mm jack on the Walkman and get a different sound. Google Play Services updated to the most recent version a few days ago, without any change to the settings. I do wish that the Headphones Connect app didn't need me to create an account for the sound personalization. I would have played around with it, but when it refused to continue without an account, laziness took over and I put it off for another day.
After more listening, I went back to thinking that the ZX707 has a bassier and warmer sound than my other gear. It synergizes very well with my MDR-Z7M2 (somewhat surprising, as I'd have thought that their similar sound signatures would be overwhelming when combined, but they aren't, the Walkman just takes the Z7's sound characteristics up to 11), and it nearly gives some life to my Aeon 2 Closed. The more pronounced bass and lower frequencies are also a nice complement to the Erco's sound, which is tonally neutral but dynamically emphasized with added punch and sizzle. This sizzle can be a bit too spicy on my electrostatics, so switching the source out to the Walkman is nice for a change of pace.
DSEE is pretty neat, and it does seem to make songs sound a bit more open (slightly and subtly) as well as adding more texture to the trailing ends of drum hits. It seems to work better on recorded instruments rather than vocals or synthesized music like EDM, and my music is predominantly EDM with vocals, so I usually just use Direct mode. But DSEE is neat technology that I appreciate. I'm trying to explore jazz these days, and maybe DSEE will help with some of the old Billie Holiday recordings.