trying to connect an Audio Technica M-2 in-ear monitor to the output of an Aviom Personal monitor, The Aviom is Head Phones out only and the M-2 is line in only, any suggestions as to how to do this without overloading the M-2 ears ?
Answering your question more directly, a headphone output is generally close enough in level to drive a line input just fine. Most pro gear with line ins can accept a signal as hot as +20dBU or more, which is more than 12 volts. Looking at the M2 manual, the M2 can accept up to a +26dBU signal.
Generally the difference between a "headphone" output and a "line" output is the ability to deliver this voltage into a lower impedance - an 8 or 16 ohm load rather than a 600ohm load. The headphone output has more current supplying ability.
In other words, fit the plugs and go.