Subdued Voices - Phase problem?

What is the distance to the speakers arer you listening near-field or far-field?

Your speakers are only designed for near-field listening. With the kind of power that you have to drive them, it is likely that your are compressing in the midrange and the woofer ringing is mucking up your lower midrange with distortion. These are great speakers but you might need better ones - given your gear. Did you move the speakers further away by any chance?

Soundstage measurements suggest a problem from the metal cone around 400 HZ at higher listening levels. Also in general this woofer might be teh culprit - woofer distortion messing up the lower midrange is all too common. (Metal or rigid type cones ring like a bell - something that becomes more noticeale when driven hard)

You gave very little information - so my wild guess could be off track. IMHO - this is a great speaker for listening at MODEST volumes.

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.

FWIW: I would consider upgrading the speakers at some point anyway (next
step). They are nice speakers at their price point - possibly even giant killers -
but your source and amplification could easily drive something a bit higher end
- especially at nine feet back from the speakers (you lose about 9 db SPL at that
distance which places you close to their upper limit when listening at moderately
loud levels - dynamics may be lost and sound may be strained/distorted)