I assembled QRV-07, it works perfect, but I need higher gain, because amp plays quietly with less sensitive models like AKG K701 and sennheiser HD 595.
I use 50k ALPS blue velvet pot at the input of the amp. Is it enough to change values of resistors R4 or/and R5 ? When I understood it right, in your original configuration is gain set to 2 - R5/R4 + 1. Can I increase it for example to 3 ?