What's the deal with this mic? It's a Shure PE66L. It looks just like an SM57. On Shure's site, I found a user guide for them and it looks like they have the exact same frequncy response curve and polar patterns as th 57s, and that they were made from'84 to '93. Anyone know what the idea behind these were? Is there something about them that makes them a budget mic or whatever?