Is the Guitar Boost feature available without using the Switch-3?
No, unfortunately the Guitar Boost feature only functions when you have a Switch-3 hooked up to your Play Acoustic.
After connecting the Switch-3, you must then assign a button to the Guitar Boost. This is done under the Setup menu on the page entitled SWITCH-3 6/7.
Once a Switch-3 is attached and a button is assigned to Guitar Boost, you will see a "GTR BOOST" show up in the bottom left-hand corner of the display when the feature is switched ON.
The Guitar Boost can provide up to 6 dB of boost. The amount is controlled with the GUITAR BOOST AMT parameter, which is also found within the SWITCH-3 6/7 page in the Setup menu.