Need help for troubelshooting a modification to a reverb circuit on a guitar amp to an FX loop circuit
Op-amp U5a is driving the reverb spring input transducer as a current source.  You have a couple of options: either convert the stage to a voltage source or add an appropriate value of load resistor so as to ensure the op-amp doesn't saturate.
The easiest might be to just add an appropriate load resistor.  The schematic that shows one person's mods shows a 100R resistor - try that.  Connect that 100R resistor between nodes P13 &amp; P14.  Take your send output from P13 (tip) and Ground (sleeve).  Add a 100R resistor in series between P13 and the output jack if you are concerned about preventing damage to the op-amp if the output is shorted.
The input return has far too much gain: it is currently about 67.7.  Far too much gain for a standard effects device.  The easiest way to bring the gain way down is to simply lift one end of resistor R40.  Still may have too much gain but give that a try first.