Are you using a voltage transformer for usa.
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It took some time to debug the problem and I think it will be helpful to share the results with "audiogoners".
First I would like to admit that Dan responded very fast. At the bottom line it did not help to resolve the problem but at least I should admit his strong wiling to help.
My source is EMMlab SACD which is fully balanced design.
ModWright 36.5 preamp is single ended design. What I found during the debug that only preamp XLR outputs are truly balance (the conversion from single ended to balance is done by splitter transformer). At the inputs balance to single ended conversion is done by simple connection of positive XLR wire to single ended input and negative XLR wire though resistor to ground. Such connections are pretty sensitive to ground scheme between the source and preamp and in some installations you can get hum noise due to ground loops between two units grounds. So if you are going to use 36.5 input XLR connection please check if you do not have ground loops (hum noise) and use good power cable to improve body ground connection between the units.
In my case the connection of unused RCA interconnect in parallel with XLR and usage of good power cables helped to reduce hum noise but I should submit that it's far away from noise level of 125db depicted in spec at least in my case.