I find the leakage bad if they're not being worn--otherwise they don't seem to leak much sound at all.
Also, I recently had a chance to compare them directly to the Beyerdynamic DT990 Pro's!
They sound very similar--the DT990's however, does feel a bit more punchy in the bass. The instruments also feel more distinct on the Beyers, where as they blend a bit better on the A900X's. Hope I can continue to keep readers informed about these cans!