As a past owner of a JCM900 series amp, maybe I can offer a little bit of help. *The 900 series, while truthfully being much less pure than previous Marshall amps, still delivers a really high gain
TUBE distortion. *If you are going for a sound somewhere between metal and rock, then I think I would suggest it to you. *Remember, it has diode clipping circuitry that shaves the top of the waveform off, and therefore creates a harsher distortion than usual. *This does, however, add to the overall gain and "edge" of the amplifier, giving it a sharper tone than an all-tube circuit. *But if you combine this with the
power tube saturation, you can get an "in-between" sound that ends up being a sharp, high gain power tube sound.
As far as flexibility goes, I do not believe in that term for digital amp modelers. *There are just not any amps that are built with mutiple exact circuitries that allow them to faithfully sound like several great amplifiers. *My advice is that you save your money and buy several amps that do 1-3 things very well rather than believing you can get a "jack of all trades" amplifier.
You can get a handful of some different GREAT tones out of a Blackface Fender as opposed to getting 40 poor but different sounding tones out of a digital modeling amp. *I would rather have 1 or 2 incredible tones than a million mediocre ones.
