You have to try the sub if you can. At first I was like no way am I going to play a sub beside me as I listen but the fellow head-fier was right it works awesome. The AD900's need a little dump in the bass anyway.
Also just purchased the HD600 and they are being delivered today any thoughts as someone who also owns the AD900 & HD600. I'd love to hear your input as the AD900 are detail masters IMHO
BTW I also own the DT880 (600 ohm)
I think you'll like the HD600. The HD600 sounds amazing, it really does for the price. All genres sound good on it too, where as the AD900 gets a little rough with metal. The HD600 doesn't seem to have the detail the AD900 has, but it's not a huge difference. The HD600 has a narrower soundstage as well, but better placement. I've found the AD900 to get a little "confused" with some recordings when it comes to placement. The HD600 is very easy to listen to, not fatiguing at all, where as the AD900 gets a little old when you're listening to music with a lot of treble. The response is flatter on the HD600, it doesn't have that weird hump in the high mids. I like that hump though for female vocals, violin, and piano.
Ultimately you need to see how you feel about them. After my recent mods, I like the AD900's more than anything else in my collection. For some though it would be the HD600 or DT880 before the AD900. Headfi is really all about personal taste and flavor. You may also have a completely different experience because of the rest of the gear in your chain (DAC, Amp, Cables, etc).