I wonder if you are perhaps overthinking this. The M16 is a very good speaker and will have a natural roll off below 50Hz. Rather than worrying about having a HPF for the M16, I would simply try and integrate the sub below 60 or 50Hz and see how that sounds. You may find that it is fine that way. If you want measurements to support the integration, you can get a Umik measurement microphone and use REW software. If you have difficulty integrating the sub, then look further into HPFing the M16 to get a higher crossover point.