I've never needed integrated Rockbox support, as you mentioned. But last.fm support on Amarok is built-in. Just add your userID / password and you're done. I've tried all the players you mentioned, and my personal favorite is Amarok. In particular, I like the plugin support. With Album Art, Amarok won't automatically pick up "cover.bmp", but you can import it manually as you listen to albums.
As for the scrobbler.log itself, I don't think there's a way to integrate that file with amarok. I use the "scrobber-cli" program via a script, whenever I connect my player.