I am also wondering if anyone has had this issue with the 10.33 (WM8741 chip).
One thing that might help is a cheap toslink-coax converter. I run an optical through my TV to my AGD DAC and it would cut out (don't know why). I had a Monoprice converter not being used so I tried that, seems to work now without cutting out.
Also, I have discovered that not all optical cables work as well as others, some seem to hold signal better (for lack of a better term).
I have an older NFB-10 and I get sound regardless of the position of the internal jumpers (fixed or variable) - you should get the same.
If you go with variable (preamp) output the volume control of NFB-10 works and you can adjust the speakers volume at will.
If you have not opened the case how have you observed both output modes ?
If you cannot find any connection mistake, I suppose you can live with that - I always keep the jumpers to variant mode.
I suppose you have tried both rca outputs and both gain settings, as well as a 2nd pair of interconnects. If the xlr output is working on fixed mode, you could check whether the speakers require more than 2,5 V input (unlikely).