I'm pretty sure it's minor issues Paul. Great to hear that for the most part your units are up and running!
Back then when I built my prototype I spent days to trace the signal through the circuit even though that included finding errors in the original pcb layout and schematics - at least they were present in the original service manual.
I have to admit it's always worth to double check the solder joints at turrets of the pcbs while tracing the signal. I have been talking to Livingnote about the pcbs and they will be thicker next time and we'll also try to find ways to improve them otherwise.
Back then, after realizing how crucial it was, I made sure I added some solder to every single fork turret and rivet from both sides.
My unit has been working perfectly ever since - even though I used 500ohm pots with a resistor in parallel for the verniers, so your choice of pots there is not as crucial as long as you use good quality pots.
I eventually replaced my "regular" vernier pots with switchable ones as I modded my unit along the way and it still works great.
I used push-pull switched pots for both verniers - the input one switches the unit to hi-gain when pulled (optional as per the original pcb) and the output vernier switches the meter from GR to OUTPUT when pulled...
There are some more mods - even ones that were offered by UA directly back then. Such as the 600 Ohm T-Pad option for input and output attenuators which - according to the service manual gives you an additional 6db ea.
The optional t-pad attenuators were Langevin AT-601 and the actual model number of the 176 would vary depending on wether input, output or both were equipped with the t-pad option.
The models were: standard 176, 176T or 176TT depending on what was in there - and BTW the list price for the base model was $480 back then, while the 176TT was $520.50
Yes 500 bucks for a 176 hahahaha
Last time I checked now was 12K...
Whatever model the unit actually was, the front panel obviously always just said "176"
The standard model mostly used DAVEN LA-353G attenuators as per the original schematics (attached below)
In my unit I had DAVEN LA-353G for both - input and output, but I eventually swapped the output attenuators for a 600 Ohm T-pad - so my unit is actually a 176T.
Anyway - back to topic. The unit shouldn't be too noisy actually - at least with my unit I was surprised how quiet it was in terms of noise or hum. If the hum is noticeable double check your ground wires and routing. In this circuit pretty much every ground wire is routed to that one star ground left of the PSU board.
But if your unit was not plugged into the balanced power and you have problems with interferences or hum in your studio anyway that might very well be the culprit for hum being introduced.
The gain being wildly different on the second unit IMHO is pointing towards a cold solder joint.
Also check your 6BC8 tubes - the first one I tried in my unit was gassy - even though it tested well on the tube tester, and also could be balanced during calibration, the signal distorted pretty easily and the meter would drift wildly in GR setting when turning the release pot...
...another 6BC8 fixed all of that.