Subdued Voices - Phase problem?

I recently moved my speakers around to equalize the distance to my prime listening position. When I found the "sweet spot" where voices sounded like they were coming from the center, it had an unfortunate side effect: the voices receded back into the soundstage, and their volume was significantly lower than sounds coming from either side. On many albums (although not all), it is now hard to make out the singer over the louder instrumental accompanyment. I have checked the phase of the speaker wires, and everything looks correct. Any suggestions?

My system is: Flac music server over Squeezebox to Benchmark DAC1 Pre to Bel Canto Ref 1000 mono blocks to NHT Classic Three speakers. Cables: radioshack Toslink from Squeezebox to DAC; Balanced Kimber Hero to amp; Speltz Anti Cable speaker wires.