the AP-2 exotic is identical to the 4th gen AP-1 exotic but adds our custom filter and push/pull knob The Archangel Ap-2 exotic works exceptionally well for eliminating feedback. We have incorporated a custom bass resonance filter nto the output stage of the circuit. When you engage the switch by pulling out on the knob, it allows the full bass response of your instrument through and the resonance filter is bypassed. When the knob is down in the normal position, the output of the preamp is conditioned through an EQ circuit to remove the explosive sub-sonic bass and handling noise, and gradually slope off the lowest frequencies. It also makes a guitar or mandolin sit beautifully in the mix with drums and bass.