elfanatic – I built an Add Verb just a few weeks ago just as outlined in the Add Verb build document. It works just fine. Barry’s recommendations are sound.
For grins, giggles and a quick little test, temporarily tack a jumper across R7. When the volume pot is turned to zero (fully counter-clockwise, the result should be that pins 7 & 6 of IC1B will be shorted together. When pins 7 & 6 (or pins 2 & 1) are shorted together, the opamp yields “unity gain” by design. The theory and documentation behind these types of opamps states that when pins 7 & 6 are shorted together, the opamp is in “unity gain”. The 6K8 value of R7 is going to provide a minute amount of gain from that opamp stage; but I also believe that the 6K8 value is used to elevate the gain slightly to recover the signal from the previous stage. So, in theory, shunting R7 should lower the output of the final opamp stage.