I don't necessaraly have a problem with evolution, see Fates Warning, they have evolved a lot (and lost many fans in the process) but I still love what they're doing.
My problem with In Flames is solely that their music becomes simpler and simpler... and they hide it by adding keyboards and sound effects everywhere, even if I don't dislike keys at all, I can enjoy some songs on RtR.
But riffs are much more basic now, drum patterns are more basic, etc.
Where are all the ideas they used to have? To me it's not evolution but regression.
Those better vocals are in fact harmonized vocals, nothing special to do in a studio, I saw In Flames live and I also heard bootlegs, the guy is not really a good "clean" singer, far from that. His vocals sound way better on albums than live, and there's a reason... it's so easy to improve your vocal performance in a good modern studio.
Don't get me wrong I have no problem with hearing clean vox in their records, I even support that, but I'd like to hear real singing then, not Jonathan Davis-like whining or distorded vocals.