In case you do decide to get an AD900 and find out it gives you too little bass for your liking, you could always try changing the earpads. I have replaced the stock velours pads with the leather pads from the W5000, and there are others here on Head-Fi who have done the same. Those pads can be had for around $70-$100 from various sources around the web. No small amount, but they do wonders for the sound, I think. I really like how the stock AD900 sounds, but only now am I completely and utterly hooked. If you think the price for those earpads is too high, you could easily go for the W1000 fake leather pads (which are still very nice), or the A1000 clarino pads. I can only vouch for the W5000 pads, but really any of these pads should give you a noticeable increase in bass performance.
Now, I can completely understand that going this route might be too much hassle or might not be worth the expense to you. If either is true, you might want to look into the closed A- or W-lines of Audio-Technica headphones. Both lines will give you more or less the same sound signature you get with the AD900: great mids and sparkling highs, but with a nice increase in the bass department. Another option might be to look into Grados. Mind you, I have never heard a Grado headphone myself, but according to what I've read they're supposed to be somewhat comparable to Audio-Technica's AD-line, albeit with a slightly less airy sound and a smaller soundstage.