Ok so i have a marshall avt 4x12 cabinet as you probably know it sucks. But if i was to change the speakers in it would it make it just as good as say a mesa cabinet. And also what would be some good speakers to buy if it would help alot. I want good death metal tone (i also use a peavy 5150 head)
The head is more important than the cab.
In the aspects of the cab, the speakers are obviously the most important. The build quality, however, such as the matieral used for the covering and the casing affects the sound too.

Changing the speakers WILL make a difference, but get another cab, it will sound much better.