Hi Alan:
This odd (to me) behavior was noticeable even after I took the VP150 off the shelf (it's not really a cabinet) and placed it where the left speaker was (essentially free space). I noticed the exact same behavior, so I doubt it was the placement.
The other center I have on hand (an NHT M5) uses a three way arrangement ( two woofers flanked by a centrally located midrange with tweeter above or below the midrange, depending on placement) and I neither heard (nor measured) any such frequency or output aberrations. Now, if you guys are telling me that these aberrations are inaudible with music or movies, that's fine. But I would be interested to see the NRC measurements of the VP150 on axis and off-axis to see if what I'm hearing shows up in as measured anomalies.
