Re: I've been listening to some older albums...
Lets not confuse gain with distortion. Even with an Old Skool Marshall, I can get ridiculous distortion with a pedal to make some amazingly fast and cool soundingl eads ( that turn to slop when I dial the distortion back down).
Some of the modern super high gain amps do a great job at keeping rythyms and leads tight even under massive gain. Thats the one missing link in my collection, is a modern high gain amp.They can be pretty unforgiving when playing them as opposed to heavy distortion, and it takes a lot of skill. No, not as "dynamic" as an old Hot Rod Marshall, but what they lack in dynamics, they make up for in speed and articulation . Punch? Don't give me punch, a mesa Recto is punchy as all hell.
Very generally:
The distortion you are talking about, comes from the sound compressing and clipping from having higher gain. Gain comes from the increased power on signal by pushing energy (overdrive - driving the signal harder and moderately clipping and compressing it) or clipping the signal (distortion - signal pushed harder than overdrive to the point of a high degree of clipping and compression).
With distortion, that automatically means a very large amount of gain. From what I know, when there is no longer any headroom from the gain being maxed (the gain may be the volume), heavy compression and clipping (solid state - diode; tube - preamp tube) occur and you have distortion.
By nature, gain will gradually lead to distortion, which by definition is the addition of outside noise (those harmonics you hear in a good distortion). Too much outside noise will made it hard to hear individual sounds, which is unavoidable past a certain point. This is the "mush" described by many in this thread.
I pretty much prefer as much gain as I can get out of a true single coil without uncontrollable noise. Too much of any signal in my band setting with 3 guitars will be a disaster, and lead only to mush.
In any mix, there are many other sounds which have to blend alongside your sound, and all should be clearly audible. The more gain a guitarist has will introduce too much signal on their part, and then the mix as a whole has too much signal and becomes mush. This is why on its own, the guitar can handle quite a bit of gain until becoming mush, but when played with other instruments, needs to have less gain to make room for others. Many metal bands nowadays have this problem, and a part of it (but not all of it) is the "loudness war" which adds way too much gain and compression. Another part of it is the guitarists not knowing any better or being arrogant by not listening to the audio engineer/sound guy.
Also - guitars should leave the low end to the bass and certain parts of the drums. Too much low end on a guitar will cause too much input for that frequency range and lead to a lot of mud. Same with any other frequency, there has to be frequency pockets for everyone so they may have there place without too much overlap causing too much input. Mids and some highs are usually the best and largest pocket for the guitar in a rock/metal mix.