Yes, it is possible to turn the speaker simulator off, please follow the instructions below to do this:

► Press Setup.
► Use the < or > Arrow buttons to move to the Guitar tab.
► Use the Control Knob to move the selection bar until Speaker Sim Enable is highlighted.
► Turn the Mix Knob directly below the display until the setting for Speaker Sim Enable changes from On to Off.
► Press Home to exit.

When turned on Speaker simulations are active. This is a good setting for use with a PA or FRFR (Full Range Flat Response) Speaker.

Turning the Speaker simulation off is a good setting if you are plugged into your own amplifier and wish to maintain this sound relative to the guitar effects.