Re: Marshall hybrids and so called 'tone monsters'
If you're a tube nut, you won't like a simple note they pump out, a hybrid amp is not a vintage this or boutique that, but if you need some aggressive, biting, raw metal sound you may like them. In my country most players are on the budget so there are thousands of hybrid Marshalls lurking around. Through these hybrids some guys sound awesome, most guys sound decent to good and some guys sound like poo.
Valvestate mkI - I think the best sounding series of the hybrid line, at least among the ones I played. If you set them right, they're killer. I can say the best things about reliability: my 1992 8080 is beaten as hell but still working. My only problem was the quality of the pots.
Valvestate mkII - I noticed a little more treble with the second generation and as my VS-II user fellows told me these amps had a habit of blowing the power transistors at near-full volume. I also noticed that they use some different Celestion speaker, I don't remember what (my old 8080 has a G12T-75).
AVT - I agree with volume problems. IMO they have a slightly smoother sound and a better tube-ish feel but at high volume the definition is not as good as the older VS models. I also noticed that at moderate volume they can produce some awesome sounds for recording.
MG - the series basically produce a colder and quicker Valvestate tone with even more bite and much more gain, with pretty good definition for detuned stuff. Despite their SS nature, I feel them much more defined than AVT amps. In my experience they (MG100) are pretty capable of handling gigs. During our 2004 autumn tour the warm-up fellow band played MG100s and they were really F'n loud. The bad side is the use of cooling vent. All cooling vents (except for PAPST brand vents) mean a sure future breakdown.
MFour - I have only a little experience with these, a couple of minutes playing and nothing more. Anyway, I've heard a lot of people playing through them and I noticed that the low response of their own MF cabs is totally undefined and spongy while from a distance they sound pretty tube Marshall-ish through a standard 1960 cab. I know 2 guys who use MFs and they say it's pretty well built and reliable.