After upgrading the preamp, decided to make the VU meters show differential output instead of single ended levels.  Single ended  noise was too high for the meters to be any use, but

The differential output was in the weeds with the preamp off.

Maximum preamp gain with no microphone was still around -79db.  The gain is only 12x.

The aux channel has pops.

Single ended preamp with maximum gain was a bit noisier.  For each mixdown mode, the VU meters have the relevant titles & the relevant number of meters.

The room noise with the microphones connected & maximum gain, so obsessing over -79db or -60db is about as pointless as any game.  

When lions 1st saw digital audio on a computer 25 years ago, -40db was the lowest useful level for any VU meter.  All the consumer gear stopped at -40db, even after minidiscs came out.   -90db was a ludicrous option on DAWs for many years.  Who knew it would be necessary for testing home made gear.