I'm onboard with JohnK.
If cds and tv are ok with the VP150, the problem is with the source or the source unit.
You could try swapping out a dvdp for another one and use a disk that had the sibilance sounds. If the sibiliance is gone, you've got the culprit.
If the dvd still sounds sibilant with another player, then you may have to consider looking at the DSP onboard the receiver as a possibility.

"Those who preach the myths of audio are ignorant of truth."