A mic pre would give you some better control over te volumn problem. Since your useing an SM58 you don't need anything real fancy - I would suggest the ART Tube MP they only cost about $100. This would also give you 48V power if you ever upgrade to a condenser mic.
If you are filthy rich, contact Martinsound on the net. They make two great mic pres -- one is expensive and the other more expensive.
But, the ART will get you started. I use the ART dual channel with a 20 db boost.
Now that I have the Yam 2816, I may have made my reverbs and mic pre obsolete. Don't know yet because I'm still learning and reading the manuals for the 2816.
cjmusicman,
The Mackies a good way to go,It will give you pre amp(s),and you can use it to mix down with.
DBX also sell a mini pre for $99,also a nice unit.