You're pretty much right.
Compared with a good speaker set-up, the TH900s LACK bass, at least at the low-end. They're generally good, and preferable to most headphones that I've heard, although I think my Stax Lambdas probably kick harder at the low end. From my limited exposure to HD800s and T1s, no thanks, they're tilted to the treble and lack bass compared with the Fostex and Stax.
Listening to something like Brand X, Unorthodox Behaviour emphasizes the point; it's weedy on the Fostex, but has lots of low-end growl and enormous slam on a high-end speaker system. However, the bass should be missing most of the time, and rattling the windows when it kicks in... it probably goes to lower frequencies than headphones can manage, though.
I strongly suspect that the perceived limitations [SIZE=13.3333330154419px]with the TH900s [/SIZE]are a source/amp issue, not a limitation of the headphones. That also applies to the mids, which are really not especially recessed, and treble, not just the bass. They are more than revealing enough to show limitations of the upstream equipment. They don't suffer from bass being bloated.