After updating to Amarok 2.8.0-r3 (making it finally work again after the breakage introduced by the update to MySQL 5.5), I noticed that both replay gain and analyzer support was broken. This is due to the usage if the phonon-vlc backend. After switching back to the phonon-gstreamer backend, both replay gain and the analyzers started working again.
AFAIK the vlc backend does not support those features.