Recording Bass Amp w/ Mic, help

[syous]

We have an SM57 mic'ing a Fender Rumble 100. Whenever we record the guitar using the same mic on a different amp...both at the same volume...the guitar soudns great but the amp sounds very muddy, hissy and really it's hard to distinguish notes. Does anyone have some guidelines to mic'ing a bass amp thru a mixer to a computer?
Thanks

[British Kid]

DI the amp. you should be able to do this if you're using XLR anyway. if not, use an impedance adjuster to get it to 1/4" jack size.