OK, in banging my head against the wall on this I tried unplugging the laptop I’m using to run FireControl from its charger. Voila! The THC voltage test now reads 172 volts (pretty darn close to the 175 volts I would expect) and during a cut with the voltage set manually to 128 volts the live voltage is maintaining 128 volts. Anyone know what would be causing this?
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A ground loop through the ground plug in the charger.
Lots of people use a plug adapter that eliminates the ground prong.
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That is a well known issue around here. I’m surprised no one asked that question from the start. Langmuir recommends running your laptop unplugged. You can also use one of those adapters that removes the ground prong, as Phillip suggested.
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I assumed the OP already tried 
It’s one of the beginning steps in the Langmuir THC guide.
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What is actually happening that’s causing this issue? Seems like Langmuir would have addressed this if it’s been this big an issue.
They answered, it’s a ground loop issue.
It’s on your end, not the table.
Langmuir can’t address your home or shop’s wiring.
this problem is rampant in the audio industry
Here’s a small video which does a pretty good job explaining the ground loop phenomenon
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If your table passes OHM’s test, your computer or monitor will probably connect the Langmuir board to the ground. The control box may also be involved. If your table does not pass, it could be the USB shield touching the case, the isolation bushings contacting the case, etc. By no means should the control box be electrically connected to anything other than the required cables. I am willing to bet that your computer is connecting the control box to the electrical service ground in your shop via its USB port. As stated above, a 2-prong non-ground adapter will most likely solve your issue.
https://a.co/d/4V7HaA2
Eliminating the ground on your computer is not optimal, as the Tin Man’s video says. If the two-prong plug does, in fact, fix the ground loop issue, a USB ground loop isolator will be the correct fix. They are on Amazon. Langmuir can send you one, or you could get fancy and buy the “BulletProof” USB isolator from Ebay.CNC USB Isolator 1Kv -(Bullet Proof USB) UC-100 USB,USB Ground Loop Eliminator | eBay
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Thank you for the explanation.