Since you already tried XMPlay, you know that it can't handle big assed databases, but it fits the bill for everything you're asking for.
I wonder, why do you want to duplicate functionality? Never tried musikCube, but for what I know it does exactly what you want, I'm not sure if it handles custom tags, but it has a SQL-based library, music playback and a separate view for streams. To me, it seems perfect.
My bets are on foobar2000. The newest version improved a lot of things, and it makes components like ColumnsUI useless (which couldn't evolve much further, anyway, there's only so much customization you can do with a columns-based interface). In my view, foobar is configured out of the box for anyone, I mean, you feed it music and it plays it. And it has a vast array of customization options for those wanting them coupled with advanced management tools.
If you want to achieve the complicated setups of some guys in HA it's another thing, but frankly, I never thought PanelsUI is worth the hard work that it demands for an app that I only take a look at during two or three minutes a day (and foobar is playing music all day).
The only app that comes to mind that it's not listed there is
JRiver Media Center, but... hmmm, that is a complete media jukebox. Not in my tastes, but it could fit you, as it's awfully powerful. Ah, yes, and maybe
MP3 Collector.
BTW, is that page yours? Looks quite interesting
