if your mic is a condenser then use the phantom power.
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SM57 doesn't require phantom power, you're fine. Outboard preamps are usually of a better quality than what you would get with a board, which you don't seem to be using, going directly into "the box" and mixing there too I assume. Don't know much about the particular model, but TASCAM has some decent preamps considering the price range, but if you really want a WOW from where you are, get an outboard pre for around $200 new. This is nowhere near the "professional" range you will read about here, but if you get one of the better budget pres, you will definitely hear the difference. Condensor mic would be next, but the SM57 should last you a bit. They are great mic, but there are some budget condensors for less money that would sound better on vox. MXL gets a lot of hype and apparently has walks the walk. Do a search. SM57 mics are great, though!