dear newfinator,
i have owned my Mark for three years, and i can tell you that i can go from jazzy clean to raunchy metal, to singing woman tones, all with a few twists of the knobs and a few setting changes.
I think if you give some better information regarding your settings, someone might be able to offer some assistance, since there are so many different parameters that affect tone on the amp beside the normal limitations of B,M,T on most amps. There is a LOT you can do to "dial" out or "in" pretty much any tone you need, not to mention the global eq.
Having said that, it is possible to create a somewhat nasally tone with the Mark IV if desired, ie class A, triode, mid gain, high mids, low treble, low bass, fat pulled, drive up above 7, gain to taste, etc. then fine tune with EQ. catch my drift??
like the man said, "Help me help YOU"
to me anybody can get a great lead tone with a Mark IV, you just have to spend time with it, it is NOT a plug and play amp, that's why a lot of people 'dis Mesa amps. I used to hate them before i actually sat down and really took the time to understand how the knobs and other functions interact on the Mark IV. Now i would honestly rate the Mark IIC+, III or IV as some of the best amps available for around and/or under $2,000.00, certainly in the top 5 for sure.
It ain't the only fish in the sea though dude, so if you find you do not wanna take the time, and it ain't workin' for ya, get rid of it, the resale is better than on 90% of the amps out there, and there is no shame in that, the Mark IV is NOT for everybody, just look at some of these replies right here, so maybe just go get a Recto, a Stiletto, a Lonestar, whatever... something that you like and enjoy be done with it. remember, playing is supposed to be FUN right???
hope you get it worked out, let us know....
peace brother,
Force