It is always a bit frustrating for classical music lovers when audio players don't distinguish properly between artist and composer or insist on sorting by artist, so that one tends to specify the composer as the artist.
Amarok is quite good there. The tag editor (right click on file or album, ->Edit track information) distinguishes between artist and composer. The collection can be browsed by any criterion by clicking on "Group by" in the top toolbar. On-screen display ("%composer" placeholder) and the track list (edit columns) can also display composer info. Only the Wikipedia context browser still has only the "Artist" option. Someone wrote a wishlist entry (Bug 147552) on bugs.kde.org - so vote for it ;-)
Note (amarok2 on KDE4.2): check the output of qdbus org.kde.amarok /Player GetMetadata. And I wondered why my Karamba edits had no effect…
Moreover, they dropped the OSD text configuration option - so no composer there either.