That's funny, cuz the Marshall MG series amp is rated as one of the worst amp buys you can make. They are muddy, CANNOT stand up to a cymbal if you're playing a band, and will not sound well with another guitar player. I've played in many bands, and tried out several guitar players over the years. They suck. Every time a guy with a Marshall like that comes in, I get disappointed and want to tell them to just go home. You can't hear anything, and it just sounds like garbage and noise. There is no distinction in the chords or notes.
The only Marshalls I ever liked were the JCM900, and the JCM800. The 800 series was made in the 80's. It was a classic Van Halen style sound. The 900 series was a 90's alternative sound. I liked that amp, but ONLY when it was at full volume... and do you know how hard it is to get a 100 watt tube amp to full volume? You gotta be playing a gig at a medium to large venue. Other than full volume, that amp too still had a horrible tone.
See, tube amps have to be pushed hard to get that warm tube sound. You have to saturate the power tubes and you achieve that by having the volume around 8-10. The only way around this is to buy an attenuator, or hotplate.... but those color your tone up a bit... and will make clean tones a little distorted.
I would suggest getting a JCM800 for the style you play. Try to get one that is 50 watts so you can saturate the power tubes... and also, try to get one that is made in the early 80's. The later 80's, and newer models have crappy caps, that affect the tone.
Also, check out Mesa mkIV, or the new mkV. They are classic 80's/90's hard rock/metal amps. It's what I eventually got. Since you mentioned Metallica, I felt I should mention this amp too, because it was their sound on the Master of Puppets and And Justice For All albums. Great cleans, amazing lead channel, and you can switch it from 85, to 70 to 50 to 35 watts, which will get you to saturate the power tubes in any venue. You can swap out two of the 4 power tubes for EL34's, to attain a more "British" tone, which would be a little more similar to a Marshall amp. The JCM800 will be close to their sound on Kill em All. The Mesa MkIV is considered to be one of the most versatile tube amps on the market.
Cabs, I don't know anything about Fender cabs or what speakers you got in it. I like a combo of Celestion v30's and 25's in a 4x12 cab. You can go to avatarspeakers.com and build your own. They're an amazing company and you can choose your own speakers.
If you're truely in love with the Marshall sound, or the "british" sound, check out Orange amps. They are far superior in EVERY way... but are kinda expensive.