I will be using an Opamp to match impedance the input impedance of my circuit with Moku output. 

Previously I was using a resistor of 1Mohm. This caused an issue where the circuit gain was depleted at high frequency, because it overrid the imepdance of Opamp (the opamp impedance is in 1M ohm range, so using a 1M ohm resistor affects the opamp TF). This issue could be seen in elog 3860.

Also, just to be sure, I am using a newly purchased Opamp. 

The voltage regulators, LM7815, 7915 are being used to take input of 18V and give opamp supply of 15V. (self reminder- I need to change the capacitor here to electrolytic ones)

I changed the Opamp circuit as shown in Fig 2.

The opamp part is good now. The TF can be seen in Fig 3. There is a uniform gain of factor 5 ish (1000/220 for inverting opamp). Hence around 12dB.