Many Icom rigs (I don't know about the IC-746 Pro) put a +8 volt bias voltage on the "MIC +" or "Mic In" line.
You _must_ block that voltage from the PR-40 with a series capacitor -- 10uF or 20uF should do the job..
There's an extensive section on Heil's website describing the interface between Heil's dynamic mics (e.g. PR-40) and Icom rigs:http://heilsound.com/amateur/tips/earlyicom.php
I believe Heil's suggested mic adapter has the capacitor built-in.
The PR-40 has a lot of low-bass response, much more than any "ham" microphone. I don't know what that will do to the IC-746 Pro's mic preamp, or compressor. A lot of people (based on reviews here) find that using an outboard equalizer to reduce the bass response (actually, raise the bass cutoff frequency) gives really nice results.
Report back -- more data is always useful.