It's to do with a linear increase in the upper harmonics isn't it? As opposed to your tube amps with real speakers etc. that are uneven, and obviously the speakers start rolling off the frequencies above a certain point naturally, with the digital stuff they just don't seem to have emulated that idea to a degree of audible accuracy.
About GR. I agree with the 'over-effected' statement. In order to pull usable sounds in it, you need to mix and match various cabs, mics etc. and voice each to their own particular range to pull an even sound. But in that regard you get something that sounds quite unnatural... I don't know... I've just never liked it. It can pull some alright sounds, but none that to me sound like a guitar being cranked through a tube amplifier.
