I mailed Bill and pointed him to this thread for some positive feedback - he replied (and I assume he won't mind me posting, since it addresses the OP question about why it sounds the way it does!):
"Thanks a lot, It's always nice to hear from people who appreciate all of the work we put into quality audio.
The tech description that someone posted is indeed rather out of date. This is a more current one:
We're all-FLAC (with a very few exceptions that I haven't been able to track down) & no longer use any overall dynamics compression on the audio in our web player or apps. Audio via the stream links -- including 320k -- is still very lightly processed. I do normalization, a bit of manual gain adjustment on portions of some songs, and a few tricks to try to recover a bit of dynamic range (& improve AAC encoding, especially at lower bitrates) on aggressively-mastered recordings. I also occasionally will do some adjustments on recordings I consider to be poorly mastered -- like 80s/90s CDs that were produced using vinyl masters (an unfortunately common occurrence)."
So it looks like the apps are unadulterated, and the stream links lightly so.
It's the variety and play lists I love, and why I listen almost everyday. The excellent sound quality is just a huge bonus on top!
Well worth a donation!