As I understand it, SM57s and SM58s have the identical capsule, so they should sound the same, although SM58s are the "conventional" cab mic. I think combining condensor mics and dynamic mics is a great technique. I use a $200, large-diaphragm, side-address Audio Technica AT3035 and an SM57. I put the '57 right into the cone, and the AT3035 a few feet away from the cab to "hear the room." I also use an inexpensive (<$200) PreSonus dual micpre. It has a 12AX7 tube plus a solid-state micpre, and you're able to dial in the mix between the two.