So here's how I do the downmix:
# Stereo Bass
Copy: RL=1*L RR=1*R C=0 SUB=0 SL=0 SR=0
# Mono Bass
#Copy: RL=0.707*L+0.707*R RR=0.707*R+0.707*L C=0 SUB=0 SL=0 SR=0
Channels RL and RR go to the subwoofer DAC. I don't perceive the mono bass as much louder and -3dB should be correct for summing two channels, right? I can switch between mono and stereo just by switching the comment statement between the Copy commands. The mono bass is more central than the stereo bass (not that the stereo bass wouldn't be in the middle), and seems more defined, but the stereo bass has more detail. Checked polarity just now, it's correct.
Yep, -3dB seems like a pretty good solution to me.