Rob Lockwood said:
As I remember the MKIV doesnt have any more gain than the III though and for me I use less of it on the IV an add a boost pedal because I find the IV gets a bit too mushy and compressed for my taste with he gains cranked up.
I'll have to respectfully disagree. But, there are so many variables that come in to play to make each amp have a foot in the other's territory, especially preamp tubes.
The Mark IIIs (particularly, the Blue Stripes) have so much gain, it's almost unusable. The Mark IVs can't even touch it. I like both amps for different reasons. The Mark IV is much darker and much more compressed (or in Mesa-speak, focused). I like to use the IV for darker, slower lead lines... Something where the tone needs to sing. Sure, it can certainly hold it's own with the Metal crowd. With that said, if you have the Blessing of being able to A/B these amps back to back, you'll hear a massive difference. With the same preamp tubes, going from a Mark IV to a Mark III, the time sounds as if a blanket has been lifted off the speakers. It's a drastic difference. You can't clean up a Mark IV to approach the tone of a Mark III, but you can make a Mark III sound like a Mark IV by goosing the bass.
Both Marks are metal machines, but can sound great for any genre of music, and they do what they do extremely well, but if I had to choose between them, the Mark IV goes and the Mark III stays.
As stayed above, some people say the Marks have a midrange honk, but if you back down the preamp controls, and start raising your master for your gain tones, the amp will lose a lot of the honk and will have so much "cut" in a band mix, you'll be surprised how responsive the amp becomes.