I have noted that Marantz likes to talk about fast slew rates and fast transient response performance in its products. That could explain their decision for setting the DAC filter to slow -- since a slow filter produces a faster impulse response and less ringing etc.. (that's even noted in AKM's own data sheet). Fast transient response could also be the reason they use HDAMs.
There may be something to this. Indeed, I can certainly feel that with music there is so much more attack/punch/drive/slam (whatever you want to call it) with AV7705 than I ever got with my previous Pre-pro -- the Onkyo PR-SC886 (and the difference is so noticeable for it to have been due to any 'placebo' effect.)
And the Onkyo's treble, whilst it was 'ice-pick' accurate, nevertheless would get very fatiguing when listening for extended periods, and would sometimes leave my ears ringing (no such problem with the Marantz). And as for the bass, while the Onkyo's went very deep, it never really had any punch to it and was actually quite boring with music. Again, these things was rectified in my system once I had removed the Onkyo and replaced it with the Marantz -- HDAMs and all