I wouldn't call that an "epic" drop in bass. There certainly is a decline in bass below 30 hz, but nothing out of the ordinary for dynamic drivers. Sure, orthos do much better through the sub bass, but the drop off at 30 hz doesn't detract from the music I listen to. Of course, if I listened to hip hop or other bass-dependent tracks, I would take offense to the HD600's rendering of the bass, but the songs I do listen to do not need much punch below 41 hz.
HD600 started to have a good rolloff starting at around 50-60hz when I listened to them. By 40hz they were very quiet.