OK, so I decided to break out the old Audix i5 today and use it in conjunction with my SM57. Is there a risk of phase cancellation, and how could I consciously avoid problems? Here's what I did: I put on a pair of headphones and chugged a chord while moving the 57 around until I found the sweet spot then anchored it in place. Yeah I know I'm talented haha 8). And did the same with the i5, luckily they had different sweet spots. Actually using this method and with the combined tones of both mics and positions I got the most gorgeous base tones I've ever gotten. I didn't find anything wrong with the individual recordings or when they were played at the same time but some interesting things happened when I was mixing. The tracks were a little boomy so I decided to apply EQ. I put a bass roll at 80 Hz on one of the tracks and everything is good. I apply it to the next track but as I approach 80Hz there comes these booming bass tones that peak in boominess when both tracks are rolled at 80Hz. I am at a loss, I have no idea why this happened or how to fix it. The resulting artifacts were 3x as boomy as the un-EQ'd tracks...which is weird.