These kind of 19” rackmount mixers are mainly used as keyboard submixer, to manage several sound sources with ease.
And in my home/project studio it happened that way, because the number of synthesizers, expanders and sound sources, in general, are continuously increasing. By the way MX-822 is a submixer of a submixer (Yamaha 01v96i maxed to 32 channels with MY8-AD24 and a MotU 8PRE) which is slaved to a MotU 828k3 via SPDIF and WC.
All that said, MX-822 does neatly its job even though to see its led meter moving I had to raise almost all keyboards output by +6dB.
Headphone output (used a Sony MDR-7506) is crisp, clear and line output must not be excessively increased in order to keep normal background noise too evident.
When powered up MX-822 produces a slight hum that is more evident if listened very close to the place where the power supply resides.
Overall I consider ART MX-822 a good product suitable for several uses.