Concerning the stuttering - it was completely away when I switched from USB-Drive to Nas via WiFi. I guess there is some extra Processor load in the Pi from handling the USB-HD instead of being nicely served by a friendly server.
In the NAS configuration the Pi even did play 192 kHz Files to my xmos/sabre without stuttering (confirmed with this great ...params command).
But this seems quite close to the limit, the output to the ODAC (which had of course be resampled to 96 kHz) was quite disrupted.

#samplerate_converter "Fastest Sinc Interpolator"

to

samplerate_converter "Best Sinc Interpolator"

Then save and restart mpd

sudo /etc/init.d/mpd restart

In my tests the processing power demanded to the little Pi was not enough to handle this better algorythm. But I was using an async device, try and see if it works for you. If it doesn't just comment the line again and restart mpd.
For more info about these different resampling methods see here:
http://www.mega-nerd.com/SRC/

Best sinc interpolator is the Secret Rabbit Code, Btw...

You were right. MPaD, the controlprogramm on my IPad, tells me 88200 Hz, cat ...~hw_params returns 96000.
So indeed in this special case there is no bit-perfect replay, depending on the capability of the dac.
That´s fine with me it´s simply greater usability, as long as I can rely on the unaltered status of the greatest part of my music.

Thanks again for the help.

cat /proc/asound/card0/pcm0p/sub0/hw_params

While playing, and it will return you the exact bit rate \ bit depth that the dac is receiving. Very interesing question btw, let me know!