I think the Green Mountian Audio Continuum speakers are easy to drive so the difference is not so apparent but I would try one setting for a few weeks (at least) and then try the other (if you have not done so already).
sorry for my earlier post OP I did not read your post completely...
It seems to me either 4 or 8 OHM uses more windings on the power supply so the quality of the sound should be higher even if tone is similar?