Hmmm, i often find the various filters quite a bit of a hassle, admittedly. However I do appreciate the option provided by Kingwa and kudos to him for the massive range of well-executed products.
I concur with yupoet's quote regarding the sonic differences between the filters.
With my V1 DI (currently up for grabs with the NFB-5 as I'm shifting to different country), everything was rather compressed. It was quite difficult selecting an optimal balance between clarity and sibilance, and I settled for filter 5 for most of my listening.
However, with the new DI, everything has opened up, and the upsampling filters are beginning to wear on me. Fingers sliding on acoustic guitar strings are now scratchy and voices aren't natural sounding, though everything is still pretty darn clear. In this case, I've just stuck with filter 1, and the NFB-5 is chuffing out clear and good sound. Although it rounds off the edges to voice and instruments, the result is still clearer than many mid level CD players including NADs and Arcams.
With the little time I have had with the amp (approximately 3 months since receiving it as a birthday present), I have decided that optimising the upstream components does better than leaving the amp to upsample and filter.
This is what I am using till I sell the NFB-5 and DI:
EAC Wav rips of CDs/ or high resolution 192/24 files -> fidelia on 96khz -> DI DSP -> NFB-5 (with DIR9001) on filter 1.
Any thoughts on the matter? Still looking for the perfect solution if there is one!