recording my Boogie dc5

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Ive been quite happy plodding along using Amplitube within my Cubase VST32 ver 5 but i wanted to try using a real amp in my recordings.Ive just bought a Mesa Boogie DC5 combo and ive got a Shure SM58 mic,problem is just before the wall of sound shoots out the amp theres a hum the mic is picking up from the amp,theres is no hum from the amp in normal playing.If i go right up to the back of the amp i can hear the hum so it isn't my PC,cables etc,anyone any ideas?I read briefly somewhere but i cant remember where about using less preamp from the amp or something?My set up is Yamaha sw100xg soundcard,gain brain preamp between mic and card(switchable from +4db to 0 to -10db)hope someone can help,thanks!