Interesting....ATH-AD900 vs AKG 701
I had recently purchased a pair of AKG Q701's as an upgrade from the AD700 (a step below this model). They were definitely more linear sounding compared to the AD700s, but they lacked something i just couldn't put my finger on. These headphones had whatever it was they were missing, and my music has never sounded more alive and "real" than before. It has a linear, accurate sound with a wide, expansive soundstage, extremely fast transient response, and the ability to pull all sorts of hidden details out of music. These headphone literally leave nothing to the imagination, it's all there, and it's all rendered perfectly and marvelously.
As far as the bass goes, i've seen many people complaining they have little to no bass. This is simply not true. They seem to do a good job at reproducing tight, accurate bass all the way down to about 30hz, below that they struggle. I won't deny that they aren't a bassy sounding headphone, because they aren't, but they definitely aren't bassless as some have made them out to be.
I also noticed they seem to perform better with an amp, however the difference is not so huge that they are unusable without one.