The Yamaha harmonizer is very good, but a little tricky and finiky.(spelling??)
The input level is the most important thing to deal with to get the best sound. Play with the level and try different settings - also try the compressor - it will smooth out some of the low spots that may drop out of the processors range.
I personally, hate noise gates - but the one on the 9kpro might help you too.
You might want to try a hotter mic than the sm58 - they are a low signal, and better suited for PA input channels.
I like the EV n/dym series - the 767a kicks a LOT of sound out for very small cash outlay!