No experience with the AD900, but this graph adequately explains the differences between the SA5K and the AD700:
http://graphs.headphone.com/graphCompare.php?graphType=0&graphID[]=2661&graphID[]=1463
First, from 200 to 500 Hz the Sony is down some, including 4 or 5 dB at 300 Hz, and this made the Sony feel even colder and thinner.
Second, the down, up, down starting at 1000 Hz and ending at 6500 Hz is in the same direction on both cans but is more severe in all three moves on the Sony, and this contributes to the Sony's midrange just feeling strange and uneven, compared to the more natural AD700. The Sony also falls off a cliff after about 12,000 Hz where the AD700 has some energy there (which probably helps the AD700 achieve a better soundstage).
The 50 Hz square wave response is not good for the AD700 but is a disaster for the SA5K, and its bass was amazingly thin-bodied even compared to the AD700.
However, in my opinion the chassis of both phones are unacceptable. The Sony was a squeaky, fidgety, microphonic mess while the AD700, despite the coax pad mod and the hairband on wings mod, was just not made to stay on my head.