In reply to:

chess, are you sure that the artifact is not specific to the DSP and its firmware used in your Onkyo receiver?

No sushi, i'm not sure of that at all.
I was testing the subwoofers at the time and made a mental note that i thought the surround sounds from some of the recordings was rather odd (again something not as easily noticed when the main speakers are playing the primary volumes).
I never did go back to sift through a couple of other recordings to see how the surround sound is reproduced with DPL2. It is possible that the tracks i had heard were mixed with this effect in mind (referring to the main stereo units, not specifically the surround speakers).
"Those who preach the myths of audio are ignorant of truth."