I would definately let Mesa know about this. I know back in the day they were very selective as to who they would let represent their products. The markIV is no good for metal???? This guy is either retarded or on crack, but it really just sounds like the same old service (non-service) you get at the guitar world type places. You have to go into these types of stores with the knowledge of what you want before hand, because the salesman will just sell you anything and they don't know squat about their own products.
I remember guitar center in Hollywood was full of these types of idiots.... you cross the street and walk into Hollywood customs (I think that was the name) it was totally different. Those guys new their stuff.
Avoid those big box stores if possible and find that neat little hole in the wall place where the guys know their sh!t!!!