that's weird. my SM57 does not pick up much ambient sound beyond it's pattern (it's a unidirectional microphone, i assume your sennheiser is too). my rode nt-1a condensor picks up pretty much everything though, so i have to have stuff dead quiet if i plan to use that.
micing the cab pwns everything, with the right amp and the right tone. heck, i even had a kustom and some boss gt-6 POS or something sounding halfway decent with the SM57. time to invest in a decent mic preamp, i think.