A customer sent us a SONOSAX SX-BD1 and indeed we found excess noise with this unit when used with a UH400a. We are 99% sure this is due to common mode noise on the Sonosax outputs particularly when the 40 dB output attenuator is selected. (We didn't have a schematic and we didn't want to tear up the customer's unit). When used with a good mixer, the common mode noise was mostly canceled out but that was not true with the UH400a.
We found that grounding the XLR pin 3 output of the SONOSAX cable to pin 1 of the same XLR removed the common mode noise by forcing the output into an unbalanced mode that matched the input of the UH400a. This can be done either in the SONOSAX male XLR itself or inside a barrel adapter available from Lectrosonics P/N 21750. This adapter is normally used to reverse the XLR polarity and is wired as pin 2 to pin 3 and pin 3 to pin 2. You can undo this by switching the wires at just one end of the XLR. To fix the SONOSAX SX-BD1 problem, simply wire pin 3 to pin 1 at the male end.