From what I have been reading jackd1 is the more stable/compatible one, and this one seems to be working (no error messages and says it is waiting for signals).
I am using the following command to start jackd1 for my usb-dac:
jackd -P70 -s -Rv -p16 -t2000 -d alsa -Phw:0,0 -p1024 -n3 -r44100 -H -z s -S -i 2 -o 2 &

I have put the following in /etc/mpd.conf:

audio_output {
type "jack"
name "my jack device"
}

audio_output {
type "jack"
name "my jack device2"
format "*:16:*"
source_ports "left,right"
}

When starting mpd in verbose, it gives no strange outputs.

But when trying to play using mpc any music through either jackd audio output, mpc outputs the following error message:
ERROR: Failed to open audio output

Did anyone managed to get jack+mpd working? Any help or suggestions are very much appreciated!