I can't speak for DFF, but I tried several DSF files yesterday on my loudspeakers and they all worked without a hitch. (This may be obvious, but did you verify you have 4.0.0 by navigating to My Invicta?)
I'll give DFF a shot at some point. Did you find a free file somewhere?
As an aside, the DSD files sound *really* good. The step forward from redbook to DSD vs. redbook to high-res PCM is much greater.