Howard,
Upon exampaning the Quavearto circuit I think what would help this issue of being so bright when a buffer is plugged into the pedal is a resistor between pin 1 on U2 and the high pass and low pass RC filters. Attached is a picture of the PCB which indicated how to make this mod. You’ll have to use your xacto knife to cut a trace and then add a resistor between pin 1 of U2 and C2. A starting point for the resistor value should be between 1k and 10k. The larger the resistor the more immunity it should provide for the brightness effect. Please note that I haven’t had time to try this yet so this is only in theory. But if you do try it, please let me know how it works for you.
The capacitor mod I mentioned could have a starting value of 1nF….which corresponds to a LPF cutoff of around 16KHz. The value of C (in farads) is equal to 1/(2*pi*R*f), where R is the value of R2 (10K) and f is the low pass cutoff frequency you want. You can recalculate for whatever LPF cutoff you want.
-Brach