I'm not sure about the 400S, but the HD650 sounds great at low volumes to me. I often will put something on rather low while I work on something else that needs my concentration.
I did own the 400i for awhile and found that it needed to be turned up a little more to get the bass response to be satisfactory.
I think its a matter of how low, my normal listening level is 65-75dB where pretty much all headphone will sound about as good as they get but for going to sleep I have the level at 50-55dB where the ambient noise level in the room is 40-45dB. This is where the elevated bass and treble of the MDR-7506 is great, it sounds over done at normal listening levels but at really low levels its like the old "loudness" switch on vintage hi-fi gear.