What's a good metal sound?

i play heavy metal music like metallica, megadeth, etc. and i have been having such a problem lately trying to find a good sound for this genre. i have the following distortion pedals. the boss ds-1 (distortion), mt-2 (metal zone), and the digitech distortion factory pedal. i really like the metal zone pedal the most, but i can't make my notes carrying on when i solo. it's like as soon as i think i need more distortion, it becomes too much distortion and can't understand what i'm playing. my notes end after about 4-5 seconds of hitting it. i'd like them to carrying on more so i can use my whammy bar on a long note (like a kirk hammet kind of thing). so is it my amp? do i need better humbuckers? i don't know what to do. for my amp, i currently have a Marshall G5OR CD. its about 7 years old now and i have moved it around quite a bit. could it be that the amp is dying on me maybe? if so, what is a good one to get? i spend anywhere from about $300-$600. also should i get a seperate head and then cab kind of deal? or a regular all in one? if it's the humbuckers or u guys think i should get better ones, what would you suggest? and if you guys have an answer on how i should set up the amp to make the notes ring out more, please let me know. thank you so much