Maybe a Vocalist or something like that is what you need, Mikey? It will have XLR inputs, decent pre's and vocal compressor and EQ, good reverb AND a decent harmonizer. FAR better than your Yamaha, at least.

Plus foot control for harmonies, reverb on/off, etc..

BTW, feedback is not JUST about gain staging. It is also a function of where the speakers are. By being behind you, the Bose system makes this a lot more critical... Possibly a good quality headset mike might help here, as your head would always be blocking the mike from the speaker behind you...?