I can't hold my breath much longer!
Ok, so I finally have an update.
I'll start by saying that I'm still smacking my head for not seeing that I had Pin 2 connected to ground. Sure enough when I put in another pair of trimmers and used a jumper from Pin 2 to the pad for R710, I was able to adjust DC offset without a problem. Thanks again to both Tom and Craig.
There does, however, remain one DC offset issue that existed in the left channel before replacing the original trimmer and is still present with the new one, albeit a little less pronounced. I can adjust to around zero DC, but it won't hold steady, fluctuating by as much as 50 or 60 mV. By contrast, the right channel fluctuates by less than 1 mV. Idle current is right at 15 mV and Here's what else I've done to solve this:
R713 - Replaced with 25 turn Bourns trimmer
C703 - Replaced with a bipolar Nichicon ES
C706, C703 - Replaced with low ESR Nichicon
C710 - Replaced with a C0G type
H701, H702, H703 - Replaced with KSA992 transistors
H718, H719 - Replaced with 1N4148
Resoldered a broken joint at the ground shield of the male RCA input plug. (I thought this was the culprit.)
I'd appreciate any direction for next steps.