One really odd aspect is how much the effect varies by recording. For instance, on any Nora Jones recording, her voice is crystal clear and near the front of the soundstage. On most other recordings, it sounds like the recording engineer lowered the gain on the singer's microphone
Another possibility is that one woofer is FRIED or is sticking or rubbing in its voice coil. This would affect male vocals more than Norah. Another issue could be phase on the woofers - if one is wired out of phase then it could sound ok except for sounds between 300 and 800 Hz - lower midrange.