Plus it'll (reliably) find and embed album art and do transcodes if you want it to.
> I yearn for the ability to use a Beets library as a source for streaming
Beets + MadSonic [0] blows everything else out of the water for me. I use DSub on Android and the web UI on desktop (although JamStash and Polysonic are good too).
This doesn't meet your "combine with other sources" requirement though. For that you'd need Tomahawk (which supports Beets and Amazon but not Pandora) [1].
[0]: http://madsonic.org
[1]: https://www.tomahawk-player.org
And EDIT: You can actually use Clementine with Beets through Subsonic/Madsonic as well.
I like Tomahawk so far for the "other sources" part (also I find the UI easy on the eyes compared to … just about anything else out there), but of course I can't get it to stream from my Subsonic collection :-/
Why do you use Madsonic instead of plain Subsonic? (Does it work with Tomahawk?)
That's really odd. Perhaps it was precaching extra tracks? I haven't had any issues with it, certainly not 1.5GB of data (although I have unlimited so likely wouldn't notice anyway).
> Why do you use Madsonic instead of plain Subsonic? (Does it work with Tomahawk?)
I found that regular Subsonic just wasn't moving much at all. Madsonic seemed to have active development.
I haven't tried regular Subsonic with Tomahawk but Madsonic certainly works for me. I also hooked it into a Beets server without any issues.
And a changelog here: http://beta.madsonic.org/pages/changelog.jsp
If you're anything to do with Madsonic development, you might think about making that comparison page a lot easier to find. It doesn't seem to be linked anywhere very much.
I'm also uncertain how "optional video player" isn't listed as a Subsonic feature, when 5.x (and I think later 4.x as well) can stream video. I mean, I've actually streamed video from my Subsonic 5 instance, so...