If it's the <$700 Marshall listed on Sweetwater, you can swap tubes, speakers, etc. to get it to sound different; you may find that buying a better used Marshall or other amp gets you further in improving the sound. This isn't a knock on this specific Marshall by any means -- its just that to hit that price point, Marshall, like Fender and other amp companies, had to make sacrifices/tradeoffs to hit that price point.
Agree with Kiwi - a 40watt amp is pretty loud; with a mic on it through a PA it's really loud. Figure out exactly what you like and don't like about your amp's tone. But it's probably loud enough.
Also helps to provide:
style of music you're playing
Band setup - are there 3 guitar players?
Guitar, pedals - signal chain
Amp settings - if you're scooping the mids, it will sound OK on its own, but will disappear in the mix -- the equivalent of sonic mud.
Live or recording?
If you're micing it, what mic and where are you positioning it
Otherwise, you will get a slew of answers that may or may not help you. More details will automatically narrow down responses to be more useful to you.
Hope that helps.
nedorama Monkey Boy Studios
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Last edited by nedorama; 11th January 2013 at 11:58 PM..
Reason: more details.