Dave,
S900, or 910 works great with TC Helicon Harmony M module.
What annoys me most about the Yamaha harmonizer is that it interrupts the lead voice each time it is engaged or disengaged.
You can make it SOUND o.k. with some tweaking, but you can't get around that on/off problem. You have to be certain you aren't singing when you hit the foot switch to turn it off or on. Also there is no light to indicate when it is on, at least on the 900.
The one in T4 doesn't have that problem btw, so maybe the replacement for S910 won't either.
A Merry Christmas to you,my OLD friend!
DonM