Yeah, the FM signal is very low power and mono. The various streams (cell phone app, web page) are stereo and generally better, but the budget means that with very few exceptions the highest-quality tracks are only 320kbps MP3. And some of those are from iffy sources and less than great encoders.
I've replaced every track I know of that has outright errors in it (and slipped in the occasional FLAC for new tracks), but there are so many in need of general audio upgrades that I've tended to slack off lately because it's just overwhelming. When my ambition is high I'll at least try to upgrade some of them that appear in my shifts, but even just there it can get to be too much.
Having said that, I'm genuinely surprised how sometimes a 320k MP3 that's been further downgraded to 128k doesn't actually sound that bad. But I'm two weeks shy of 62, so take that with a grain of salt.
If you're using a Squeezebox or anything else that will take a raw URL, you can listen via
http://janus.cdnstream.com:5160/live. We're coming up on the sixth birthday and that URL hasn't changed yet, so fingers crossed.