This commit fixed multi-channel JACK support, although it likely introduced another problem (at least on my ALC1220 hardware on Fedora 33): when JACK is selected as audio output and number of audio groups is forced to 2 (the multi-output checkbox is off), all the even-numbered channels (2, 4, 6, 8, 10, 12, 14 and 16) are not heard, despite MIDI activity is being shown. Odd-numbered channels (1, 3, 5, 7, 9, 11, 13 and 15) play back normally.

Workaround is to manually edit the Qsynth.conf file and force AudioGroups setting to 1, and after that all channels are heard.

This is probably related to commit above, since I've started experiencing that only after updating Qsynth to 0.9.1.