The Phantom Octave is an octave/ring mod type fuzz that's very interactive with guitar pickup selection and volume/tone controls. The LED pads are for the one that's under the knob in the video below. You could use it as the on/off indicator, but it might be hard to tell if it's on when signal isn't passing through it. Should be able to squeeze it in a 1590A. From the manufacturer: Equal parts mysterious and ghastly, the Phantom Octave is a shape-shifting octave fuzz monster! No controls? They're not necessary--they're at your fingertips! The Phantom Octave is extremely interactive with your pickup selection, picking intensity, and your guitar's Tone control. Like its namesake, the octave will disappear with picking intensity and by rolling back your guitar's Tone control. Octave effect is strongest with low-output single coil pickups.