IN MY OPINION
I totally agree with Bob. Simulated guitar sounds are terrible to mix. They just wont 'come forward' properly. It's as if they dont move the cone as much as a speaker captured with a mic. And for me there's the rub. It's not the simulation of the amp that's so wrong (although I do believe a great real amp is better than a great simulated one). Because if you run simulated sounds at high level into a real cab you can get sounds that work properly. OK they might not be quite what you expect...They might in some cases be better but the component we dont yet simulate properly is the speaker, and to be more specific the speaker distortion. The difference is that with the real speaker, as it distorts, in addition to distortion we get a limiting. And the charachter of this limiting is that it seems to allow highs, mids and lows of the sound to push up against the end stops. To align. To glue toghether.
I've recently been playing around with running the unsimulated output of a dummy load connected to a valve head into speaker impulse responses. Invariably my first reaction is 'wow this sounds good, It's so cool that we can do this'. And the second I then switch out the real cab I feel a wave of disappointment as I realise how much better it is. Every time. I believe the problem is probably to do with the fact that the sine sweep just cant make the speaker extrude in the right way.
Of course this is a bugger, because the one thing a lot of us want to do a lot of the time is get a really great 'silent' guitar sound. My experience of this is that at the moment at least, you can't.
But I do believe we will see this as and when people turn increasingly clever technology at the problem of speaker simulation rather than amp simulation. I was rather hoping Anamod might have a crack at it. I did email them about it.
NB. I'm not saying that people don't use these sounds on records. Fake amp to real speaker gets used a lot and has done since the 80's. Fake amp with simulated speaker gets used too. But I agree with Narcoman...These sounds are not great. I don't believe they're on Slash's record, put it that way.