Perhaps that's something specific to SPDIF, in which case I wouldn't know --- I've only used the HP exclusively with USB, and in-between tracks, unless it's switching between filtered and bit-perfect (red book and high-res), I don't hear the fade-in effect. Personally, I like the fade-in effect, but I can see how it could be distracting if someone listens to a continuous album.
I didn't notice it at all with USB to be honest. If it's a 3db fade in I don't think I'd be quite so bothered, but this is fade in from nothing so the start of the song is inaudible. I plan on using the Concero almost exclusively with spdif and that works really well in all regards except for this. If I had to guess I would say the Oppo is muting the output between songs and the Concero fades in each time a signal is first detected. I think I've seen that before, but with completely different kit.