Cass - Use the Harmony unit direct from your mic to the KB input, but watch the gain structure ... avid too much gain, and use the yamaha compressor - it'll save the circuitry. The XLR on the Bose compact is a teeny bit noisy for my taste, but you can use all three inputs in the other channel at the same time.

For me - the best scenario is always to have the vocals monitored from the KB speakers in my face, so I'd give that a try first. Yamaha harmonizers have always disappointed me, so I support your choice of the TC unit and I think you'll be happy with the result - just start at zero mic input gain and work your way up slowly .... you'll get a sound that you like!
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