In general we would recommend that you use XLR wherever possible in preference to 1/4".
1) It is the same with the 7600 if you use XLR.
2) Ground loops are created by misuse of pin 1, not 2 or 3. Depending on the design of your rig (beyond the scope of this forum) you may want to leave pin 1 disconnected. The main difference here between the H3000 and the 7600 is that with the 7600, both pins 2 and 3 need to be connected to something.
3) If the grounding is wrong you will probably get hum problems whether or not the 7600 is switched off. If it is right, there will be no issue.