People have measured the relative volume of OPL channels compared to PCM channels on SoundBlaster and compatible cards and concluded that OPL is about 8.5 dB more quiet. This is for a single OPL channel in additive mode running both operators at 0 attenuation. I found these results here: https://www.vogons.org/viewtopic.php?f=62&t=49683&start=20
The expectation would be that ScreamTracker matches this pattern when both master volume and global volume are set to their max values of $7F and $40, respectively. I have confirmed this with ScreamTracker 3.21 in DOSBox. This means that for a master volume at the default level of $30, which scales PCM samples by 3/8 or -8.5 dB, the volume of PCM and OPL instruments should be nearly identical.
When measuring the output of OpenMPT, I found that the volume of OPL instuments is 6 dB higher than expected, i.e. at the default master volume, OpenMPT has nearly the same volume for PCM samples and OPL channels with only one rather than 2 additive operators active, or equivalently channels running in FM mode. I have only measured the output of OpenMPT by using the "stream output" feature, exporting to a WAV file.
I am assuming that OpenMPT intends to match the characteristics of ScreamTracker in this regard (as IT and others don't support OPL instruments) and that the objective is to replicate the characteristics of SoundBlaster cards to a certain extent (otherwise it would make sense to disregard the master volume in S3M files, which only affected playback on SoundBlaster).