Dynaco PAT 4 Pre-Amp question


This pre-amp has been modified with volume control in the place of the input connection.
I have found out that when I use amp outputs on back of unit...
 Output 2 has a slight hum.
Output 1 that is modified (with volume control )...there is no hum...quiet.
Does this explain the reason output 1 was modified ?
Thanks
rocky1313