A big part of the feedback issue is the use of two Bose towers. The coverage is so wide that your "safe zone" from feedback is greatly reduced. This is a strength of the Bose design, but works adversely if you're using a harmonizer. My advice - Place the Bose in front of your mic, and use a monitor, if needed/wanted.
Also, use a good quality Super or Hyper cardioid mic to reject sound input from the sides. Set your mic gain so that your normal volume requires you to be right up on the mic. Kissing it is where I like it. Keeps me grounded, and keeps out unwanted noise from around me. Favorite mics for me are:
Sennheiser E935/945, and EV 767/767a (didn't love the 967 - hated the flat top, but that could be changed out, I imagine)