The lower impedance of the VP150 should not affect the sound quality, as long as it is properly calibrated with respect to channel level and delay. As long as the receiver tolerates the 6-ohm load (meaning, exhibiting no thermal or over-current shutdowns), the speaker impedance itself cannot be the cause of the alleged sonic "problems" if any.
Incidentally, my VP150 never prominently "presents" itself -- the only way for me to tell that it is working is to move away from the "sweet spot." Without the VP150 engaged (and in the phantom-center mode instead), the dialogs will collapse to the side. With the VP150 active, the collapse does not happen and the center image remains anchored correctly even from off-center listening positions. And this is essentially the only thing that my VP150 provides.
I would be unhappy if I actually "hear" my VP150 working.
