Hmmm...dialog normally comes out of the center channel, but you're saying it didn't help much changing it? Do you incorporate your M60's in your theater system or is it just the Quintet's? If you do in fact use your M60's as the main left and right speakers, then maybe you could try goind without a center (a 'phantom' center) and let the M60's fill the center themselves. Just a thought!

M60s
VP150
QS8s
Marantz SR6003
Samsung LN52B550
Oppo DV-980H
Insignia NS-WBRDVD