In a thread on the Metallica forums, user Hetfield1963 says he conducted a phone interview with the band's James Hetfield, and quotes:
JH: I think things came out really good. They’re going to be mixing it while we’re away in Europe. Yeah, and that will be…well,we haven’t done that in awhile. We’ve usually been around for the mixes. I think it will be good for us to step away from it for a while. Let Greg Fidelman do his magic, you know that he is the engineer, he’s going to be mixing it. He is very tuned into what we like and need. He’s very tuned into what Rick likes and needs. So, he’s kind of the point man, which will be really great. We don’t have to worry about what Rick is going to think about this, or Rick isn’t going to have to worry about us battling him on some thing. You know, [Greg], he’s going to be doing the mixing, he’s a guitar player, he gets some great sounds and that’s really helpful.
Elsewhere in the thread, it is also claimed that the band weren't present for the album's mastering sessions, because they were performing at Ozzfest.
The engineer mentioned in the thread, Greg Fidelman, shares a controversial history with the albums producer, Rick Rubin. They both worked on Red Hot Chilli Peppers' "Californication", another CD both revered and reviled for it's sound - harsh, highly compressed and very loud.
Sound familiar ?
The question is, at what point did the sound of the CD move away from the comparatively clean, dynamic "Guitar Hero" version, to the highly compressed, distorted final release ?
Update: Rolling Stone magazine is reported to have an article with Lars Ulrich, saying
the record, more than two years in the making, still isn’t finished. Rubin is overseeing mixes in Los Angeles while the band is in Europe, headlining shows...