Very sorry for late and I'm modding 8 fi.Q's from yesterday and struggling to make guidances.
The switches on above side will set for......
SW1; BUF634 Wideband mode ON/OFF with 200 ohm connection btwn 1-4 pin.
SW2; Output impedance changing switch (aka A47), and will improve stability for unstable OPAMPs,
also changes proportion of output currents from buffers and OPAMPs.(Highly recommended to set this ON)
Output current proportion
When ON, OPAMP : BUF on front : BUF on bottom = 1 : 1 : 1 (OPAMP works 1/3), can achieve less distortion, stability.
When OFF, OPAMP : BUF on front : BUF on bottom = 1 : 5 : 5 (OPAMP works 1/11), can provide more driveability for low efficient headphones.
If you mind the hiss, are you using with default setting?