Ian,

Our various IEMS have output in the 114 to 124 dB SPL/mW sensitivity range.

Frequency ranges vary slightly, with the lowest down to 8 Hz, upper range up to 20 kHz.

Now, these are tested in a rig designed for hearing-aid/IEM test rig, where the ear canal is simulated by a 2cc coupler to approximate ear canal acoustics when the canal volume is somewhat taken up by a hearing aid or IEM.

I would suggest a hardware limiter on the output before going to the IEMs, giving you some control of what the IEMs experience for signal level.

-Scott