The Zoom or similar will work. The H4 (I think) which I have, has stereo mics, and two XLR inputs, with separate volume controls. The simplest option is to take a DI feed from your bass amp or a different pedal, plug into one of the inputs and mis that and the stereo mics, plugging your IEMs directly into the headphones out of the Zoom. No need for a P2. If possible you can take a feed from the desk and plug that into the other input and mix as little or as much as that into it. How effective it will be will depend on how effectively the IEMs block out ambient sound from the room.