How does the vocal Harmony function on the Boss Acoustic Singer amps work?

This ingenious feature actually takes its harmonic information from your guitar chords and throws up harmonies that are impressively convincing and perfectly in tune (as long as your guitar is!). You can dictate the general position of where the harmony appears by choosing High, Unison or Low from the front panel as well as choosing how loud the harmony is compared with your lead voice with a Level knob. We suggest having the harmonies sit behind the main vocal for the most effective sound.

How does the anti-feedback feature on the Acoustic Singer amps work?

The Acoustic Singer uses what is called a 'notch filter' to get rid of annoying feedback frequencies. Basically, as you turn the anti-feedback knob, you are moving through a wide band of frequencies, all of which relate to the acoustic guitar. Whichever point the knob is aimed at takes away that frequency altogether. This is the notch! So, if you hear a feedback squeal, turn the knob until the notch sits over the offending frequency - you don't need to know the frequency, just turn the knob around slowly and keep your ears open - the notch filter will eradicate that frequency entirely, thereby eliminating the feedback!