Wow. Quite a mixed bag of good advice and well-intentioned misinformation here.
- the SM57 & 58 are far from flat nor should they be. If flat response was all that importan, we'd all be using omnis anyway. They are also indentical save the screen on the 58.
- the freq resp graph of the 57 posted is measured at a distance that most would never consider for bass cab pickup. Used up close, the 57/58 will exhibit a significant bass boost due to proximity effect (as will nearly all directional mics - the EV RE20 & RE27 being notable and germane exceptions). Plenty of sensitivity down to 50ish Hz when close micing.
- if you're blending with a DI, almost any mic can be used. If you rely primarily on the DI, you're just mixing in color and character from the mic. I have my favorites (Beyer M88, Audix D4) that can stand on their own, but I've never had anything unusable turn out recording with even a $30 DI and any available mic. Do some reading on placement techniques and get an ear for what does what and you'll be fine.
- don't worry too much about how extended the LF response is in this situation. The Yamaha Subkick is well accepted among folks for massive low end impact, but just take a look at the published FR graphs. Not much different on paper than many, many vocal mics.