The Fisher Bodypack ist exactly that, what it seems to be: A road-fit, passive Headphone volume setting system.
I’ve used it now several times on stage, and regular in the rehearsal room. The quality is VERY solid. I don’t see, how this thing can be damaged in any way. The belt-clip the jacks are very strong.
The Bodypack is passive, so it can not enhance the volume more, than the preamp in front is already giving (so the volume-knob can only decrease volume).
I use it for in-ear monitoring, when I don’t want to use a wireless system, and up to now, it has been a great part of my gear (Even saved my a** once, when the wireless went down).