Do you enjoy the FF mixes equally the same as the ES mixes as far as vocals go?
I don't think this has anything to do with systems, just personal preferences. I'm just stating mine and why I would rate this no higher than a 7.
I need to A/B some of the ELS songs with the FF to say for sure. That will be a fun experiment.
As far as system vs. preference goes, I think that can be true and probably often is.
I'd just throw out the caveat that with lead vocals in the surrounds, the engineer is taking the chance that certain listeners either have their surrounds over- or under-emphasized, compared to the original studio monitoring setup.
Backing vocals tend to have a volume discrepancy of 3 to 6 db or so, from lead vocals, so I think differences in calibration are not going to be noticed with backing vocals in the surrounds very often. But when the volumes should match, I think more listeners are going to notice. On my system, for Hunger Stike, Vedder and Cornell sound more or less equal in presence and I find the mixing choice kinda cool. I can understand how it can easily sound wonky though and it was a risky choice. That's just my understanding of things though and I'm open to others' theories!
Now, that only speaks to the criticism I've read about HS where the listeners have stated the vocal presence of the two singers sounds mismatched. I wouldn't like that either. At all. But, if there are listeners for whom the two parts are balanced and they just don't dig that Vedder is rearward and Cornell up front, that's cool.
IIRC, I have a Monster disc full of duets where the star singer (Ray Charles?) is panned up front like 30% left and all the guest singers are panned up front like 30% right. Now, that's an odd choice, but kinda makes sense spatially. Two singers dueting might not want to stand right on top of each other, but might sort of face each other from opposite sides of the mic, you know?