I can't always distinguish Addictive Drums from BFD from Superior Drummer from SSD, but I can always identify Session Drummer in a recording. "Dull" would be the adjective that comes to my mind, but I guess "dark" is kinder.
I suspect a lot could be done to brighten them up and make them more dynamic. Heck, long ago - long before Session Drummer came along - I was using the TTS-1 for drums. Some of those old projects sound pretty decent, but I remember that it took a lot of work involving transient shapers, EQ, delays and layering. I think nowadays the bundled Percussion Strip would be a good place to start beefing up Session Drummer tracks.