If both inputs of your instrumentation amp are connected to half the supply voltage and its gain is set to 1500 or less, then its DC output should be near half the supply voltage.
Are you using shielded audio cables connecting the probes to the circuit? The shield should connect to Ref at the circuit and not connected tto anything at the probes end. Then they aren't antennas.
The circuits with a dual supply polarity are the same as this one. The Ref is at half the supply voltage.