On current openMSX, nearly all emulated MSX that have a built-in OPLL seem to have copied the same (great un)balance between the volume of the PSG/OPLL as featured on the Panasonic 2+/TR machines.
IMHO, Panasonic produced wonderful machines, but definitely their sound is not a good reference for the volume mixing between the slot, PSG and OPLL. Sony clearly had a much greater care about the quality of sound on their machines.
I have one Sony HB-F1XDJ and there's no big discrepancy on volume of the OPLL and the PSG like my FS-A1ST has. The Sony machine has a much better equilibrium between the two, and the result is that games that play music using both PSG and OPLL are much more pleasant for hearing on this machine. Also, the OPLL has a much crisper sound, unlike the dull FM heard on Panasonic machines.
In fact, the Panasonics nearly ruin the chords and the Sonys sound like a dream when compared to them: Not only the sound is much clean/richer, but the balance between the PSG, OPLL and SCC (slot soundin, in other words) is perfect.
If you need to, I can post recordings of the sound output of both machines so you can hear what I mean. :)