Live sound and (fixed location) installed sound both have one identical issue: the need to track, trace and hear audio on various lines. Carrying a cheap, high-impedance headset (2k-ohms or greater) with alligator clips as a plug-on adapter is a sure-fire way of listening to line-level signals, as well as attenuated speaker feeds. Adding a mini-box with a transformer (impedance step up for higher listening level to +4dB lines) and a resistive loss pad (about 50dB factor for safer evaluation of speaker feeds) will further enhance the serviceablity of your 'phones. Cheaper by far than a fancy amplified belt pack, and more robust, too.
Allan Soifer, freelance mixer and consultant