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Regarding performance, the m9XX delivers. Tone is very neutral and even from top to bottom, with no undue emphasis on any frequency range. The sound is smooth, grain-free, and dynamic. The noise floor is vanishingly low. Prior to owning the m9XX I had a Schiit stack of the Bifrost and Asgard 2 (also a very good setup), but I'd say the m9XX gives up nothing in performance to the Schiit and in some ways even betters it.
The m9XX has an amazing ability to drive anything I throw at it. I've used it regularly with low impedance headphones (such as the Sennheiser HD598 and Fostex TH-X00), high impedance headphones (such as the Sennheiser HD600/HD650 and Beyerdynamic DT880/600), and high-sensitivity IEMs (such as the 64 Audio U4-SE) -- the m9XX handles them all with aplomb. The 600-ohm Beyerdynamics in particular can sound flat and lifeless on many other solid state amps, but the m9XX makes them punchy and dynamic, elevating them to a level of performance I've only been able to get with OTL tube amps in the past. I find that the digital volume control works great for the high-sensitivity IEMs, much better than the analog potentiometers found in most other amps. It allows you to make finer adjustments to the volume with none of the scratchiness / noise you can get with analog pots.
The m9XX offers two options to power it -- you can run it from your computer via the USB cable, or you can run it from a separate USB cable plugged into an AC adapter to get additional power output capability. With all the aforementioned headphones, I've never once felt the need to connect the AC adapter for extra power.
Many other headphone amps can double as space heaters with the amount of heat they produce, but I'm happy to report the m9XX runs cool and is very efficient.
Regarding simplicity, the m9XX is dead simple. Power up your computer, and the m9XX powers up along with it. Two headphone outputs on the front, one RCA output on the back, and a nice volume knob on top. The menu system (which I've only used once when first setting up the unit) is easy to navigate and well-explained in the owner's manual. Cables are kept to a minimum -- one USB cable from my computer to the m9XX, and one set of RCA cables from the m9XX to my speaker amp. Nice and clean.
I have only one complaint about the m9XX -- when I want to use my speakers, I have to unplug my headphones. The m9XX is designed so that it can be used as a preamp, so the RCA output level is also controlled by the volume control. You have to turn the volume up to 90 to get a proper 2V line level on the RCA output, but 90 is way too loud for any of my headphones and could damage them, so I have to unplug them. It would be nice if either:
1) There was a menu option to disable volume control on the RCA output, so that it stays at a fixed 2V, or 2) The headphone outputs could be muted while leaving the RCA output active.
That is my suggestion for Grace Design if they happen to read this. Otherwise, I find the m9XX to be beyond reproach.