figured it out.
The manual (29.3) describes how to do it in a somewhat confusing manner.
"Selecting one or more channels is done by clicking on the white label which turns orange then. A click in the third row with pressed Ctrl-key activates internal loopback mode, the label turns red."
Actually - Selecting one or more channels is done by clicking on the white label which then turns yellow.
Control-click in the Hardware Output row (3rd) on the Channel label, turns that label red and activates internal loopback mode.
Then Options/Input Setting in digicheck and set it to whatever channels you have chosen for Loopback, brings input to digicheck.
In my case, I Matrixed Outs 1/2 to HW Outs 1/2, 3/4, PH L/R and SP L/R.
HW 1/2 feeds the digicheck input via Loopback as well as my main monitors.
HW 3/4 feeds my alternate speakers
SP L/R feeds an outboard DK meter that I was comparing to digicheck.