You may connect an external preamp to FX Return which bypasses everything but the Reverb and Master controls. The external source will mix 50/50 with the internal preamp, which allows for easy comparisons by swapping inputs after dialing each in.
Using the FX Return also bypasses our internal clipping circuit, so nothing limits the amp from exceeding its intended 200W maximum power. The amp will shut down if grossly overpressed, but your speaker is likely to fail first. So be careful with volume levels.
If the FX Return signal seems rather low, you can boost it by putting a dummy plug in FX Send, which disconnects the internal preamp and raises the input impedance of the FX Return.
If you would like to retain the calibrated peak power limits shown on the Master, as well as using our rich chewy overdrive, you can connect a preamp to the regular guitar input.
Check out this video to see it being demonstrated.