Update to my update:
Got this message from Doug last night:
"If the 706 proves capable, I'll use a neutral sounding op amp for the TRS as it's most likely to be used for a board feed. The stock chip, SGM8272, has a dynamic range of only 80dB. That sucks !"
If this additional option proves workable, I've asked Doug to go Warm Mod using FETs for channels 1 & 2, Concert Mod for 3 &4, and whatever neutral op amp chips he ends up selecting for the TRS inputs on 5 & 6. That way I can still run 2 pairs of mics into the XLRs with a SBD feed going into the 1/4" inputs that can accept either a balanced or unbalanced feed.
I guess my unit will end up being a Guinee pig for these upgrades, but I'm certain the final result will be well worth it. This is probably going to be the last portable recorder I'll ever buy and after almost 34 years of taping and gear purchases along the way, I can't think of any other deck I'd rather have in this position.
Merry Christmas!!!