Sounds a bit strange, shouldn't sound any better unless the analog outs from the STB are pretty crappy. Most of the DSPs are garbage and just for goofing around, the 'Straight', 'Pure Direct' or 'Stereo' settings are usually what I'll use for two channel listening.
f you have your CD player in the same configuration (digital coax/toslink and analog RCAs), do you notice a difference switching from one to the other? I've got my 2500 hooked up to my Yamaha C750 universal changer w/both toslink digital cable and RCA, but the sound from either is identical regardless of the CD source.
May want to consider trying this setup (if possible from your CD player) to see if you get the same affect, this would either indicate the analog outs from the STB are garbage, the receiver is affecting this behavior or possibly something else is at work here... -TD