The Final Drive is a transformer buffer stage with several benefits:
1) Isolates the DAC or preamp ground from the amps galvanically
2) Creates a true balanced signal to the amps - solid-state cannot do this
3) Enables other sources, such as Home Theater processor or Vinyl preamp to be added, even when the Overdrive SE drives amps directly
4) Enables balanced XLR signal from DAC to be converted to single-ended RCA or visa-versa. Enables highest quality playback using the Overdrive SE (balanced) to the best tube amps (RCA).
5) +6dB of gain can be selected for any or all inputs.
However, the Final Drive was conceived not for all of the functions above, but because of the significant improvement in sound quality you get with it. Even your Home Theater will sound better using the Final Drive.
It consists of two chassis, one for right and one for left channel. Each has a selector for three inputs. Outputs are XLR or RCA, selected by the front panel switches. It is either-or, not both.
Front-panel does not have finishing steps applied yet: