The secret to e.q.ing the 855 is that it needs VERY LITTLE e.q. I used it successfully on both Tyros 2 and S900. You will probably have to roll off the bass band a little, and leave the mids and top flat.
I have no problem at all with the vocal processing on the Yammies, but the harmonizer is very sensitive and must be adjusted quite a bit. Roll the harmony voice back to 70 percent or so. Most of the garbled harmonies come from too much bass e.q. on the mic. It's never going to sound like a really good harmonizer can, but you CAN work with it, as I have done ever since they started putting them in, beginning with PSR 800.
It's very easy to fall into the old analog habit of using the smiley face e.q. The new digital stuff rarely needs that radical e.q.
DonM