Question

I am recording main mix to ch 15-16 via playback of individual channels and recording of 15-16.  But when I play back 15-16 sound is muffled and weak.

Answer 1

Bus recording on the StudioLive mixers is always post fader.

If your main bus audio signal is low (or weak as you state), then you need to bring up the main fader while recording the main bus.