Re: M as a USB DAC
In fact, Dr. Yi, the CEO of D&A, has an opinion that the M as a USB dac is better than the 24/192. I myself had used a 24/192 before buying an M and I strongly agree with him about this, but there are some conditions.
The most serious obstacle to getting full(?) potential of the M as a DAC is the PSU of course. I tried gd-audio's class-A PSU, and in terms of sound it sounded great, but because it is a shunt regulator with limited current capability, it not only couldn't charge M but also drained M's battery used with the M as a USB dac. Now I'm using a sigma 11 PSU of AMB Lab (DIY assembled). I think the class-A PSU was sonically better so I'm considering building a shunt regulator with 1A capability using the Salas' board, but it will be a big project to me and I don't have time right now. It should require a huge heat sink, and casing will be quite a job. Anyway a linear PSU benefits the M (as a dac) greatly.
I'm infusing the sigma 11's power into the USB line from the Mac mini to the M with a DIY connector following a recipe I've encountered at computeraudiophile.com. (I don't find its url right now, but its idea was breaking +5 and gnd lines of USB connection and deceiving the computer's USB port by inserting an LED between +5 and gnd.)
Being a source component, the M's output jack is connected to a 3m Y cable. I made mine with the blue dragon bulk cable from moon audio and a pair of Eichmann bullet plugs. 3.5 mm plug is a switchcraft I think. Anyway I fed the April Music HP100 headphone amp with this cable driving the HD600s. Originally I used HP100's volume and the rig sounded very good, but I tried M's volume control putting HP100's volume to the max.
Well, I've got some hum. Usually forgivable (at the max volume), but it is a problem if I were to use the M as a preamp and the HP100 as a power amp. And I believed this configuration sounded better except the hum. At that time I grounded the power inlet of the HP100 only in the rig. So I hasn't thought that the grounding was the problem. I accidentally cut the ground to the HP100's power and the hum disappeared. But without any ground in the whole rig, it sounded worse... I thought that I should ground somewhere else, and I noticed I connected the shield of the blue dragon cable to the ground pin of the 3.5mm switchcraft plug. So I can ground there I thought, and bingo! The hum disappeared. The sound was great. With this setup, the images presented by the HD600s are completely out of head and very very holographic...
With this setup, I think the M as a USB DAC completely exceeds the 24/192. I strongly believe it could compete any audio grade DAC under at least $2k. And now the M (with same PSU and USB connection) is feeding the Ayre V5 driving the Mini Maggie system with an additional DWM woofer. This stereo setup presents a spectacularly deep sound stage and distant stereo images in my small room (less than 10" x 15"). I'm very happy with my new speakers.
Looking at the attached picture of my M with the PSU and the USB connector, you would understand what I did.