Regarding voltmodding these units, R1 and R2 are the ones that give feedback to the power controller chips.
As you can see by looking at them, the boards are splitted in 2 halfs, each half having a separate power module (1 module for 16 hash chips) so when modifing R1_1, same modification is needed for R1_2 as they have the same values. The same goes for R2_1 and R2_2.
Zeus chips can operate from 0.8 volts for Vcore upwards ( I did not try to overvolt, so max value unknown).
The interesting thing for me is that these chip can be undervolted/underclocked to require only 3 amp/blade (12-13amp/1complete unit + fans) while hashing at about 2.8-3Mh/s per blade or 11-12Mh/s per unit. This would mean a power requirement of about 13watts for 1Mh/s at minimum as oposed to over 30 watts/Mh from factory.
Also, the generated heat would be almost neglijable at these settings. Probably only using one of the units fans would be enough.
If a modding and/or service tutorial would be usefull for you, please let me know.