The Boom's PAD switch is special. Not only does it reduce the input gain, but it also greatly reduces the input impedance.
Normally, the Boom's Instrument Input impedance is 1MOhm, suitable for the Hi-Z passive pickups on guitars and basses.
When PAD is engaged, the Instrument Input impedance is reduced to 3.2kOhm, suitable for modern keyboards and the Low-Z Active Pickups found in guitars and basses.
If you plug a passive instrument pickup into the Instrument Input and engage the PAD, the sound of the instrument will be radically changed given this alteration in impedance.