I got it to work.

I had the output from PRINT STEM going to S/PDIF per the D-Box instructions, but that didn't work. I could not find the "S/PDIF monitors Outs 1&2" checkbox anywhere in Console settings, so in Logic I changed the output of PRINT STEM to Stereo Out. Bingo! I had the slapback again. As soon as I muted Inputs 7 & 8 in the Apollo Console the slapback was gone, and I am now able to monitor the summed output going thru the mixbus plug ins. Dang! I am going to have to document this and print it out, my tired old brain won't hang onto this if I don't.
Thank you guys so much for your help!