Long story short: I think I have (accidentally) found a work-around for very scratchy audio quality from the Spotify desktop app. Basically, if the volume control in the app (which seems to be equivalent to the "Spotify" input control in the PulseAudio mixer) is set to indicate (per the mixer) say, just slightly under 100% - e.g. I think 97/98% may be enough, but have been trying at 95% today to give it a bit more headroom) then most tracks sound significantly better.
It could be coincidental of course - as it does seem variable day to day - I'm just guessing sometimes I've accidentally hit the volume in the app with the mouse wheel etc. So I'm wondering if it's been clipping the audio when set at 100% in the app? Note that it doesn't seem to be the overall volume that's the issue - i.e. if the in-app setting is 100%, a track that's affected sounds awful no matter what setting the main PulseAudio volume control is set to!
A few more details - app version currently 18.104.22.168.gf614956d but same effect with earlier versions; output to UA-4FX is operating at 96kHz/24bit (set by hardware switch). It sounds great when playing local files via Clementine, and other sources, so I'm not inclined to think it, or subsequent audio system is the problem, but was wondering about resampling etc. I've turned off the "normalize" setting in the app (having seen reports it may cause issues too, but that didn't seem to be the solution).
Also note that some tracks seem to have been more affected than others - I'm now guessing that'll depend on where their peak volumes are …
Interested to know if anyone else has this issue (or is it just some quirk with my particular setup?) If it's general, would be good to get the app tweaked!