Good thought, Adam. So many great points in this whole thread. My evolving understanding is that the things which affect sound quality are quite different on the two sides of the DAC. The digital side is all about latency; the analog side is all about distortion.
On the digital side, latency (the time between commanding something to happen and the time its effects occur) rules. How long does it take to read a block from the NAS, or the disk, or memory? How long does it take to decompress the block? How long does it take to apply DSP conditioning to the block? How long does it take to transmit the block to an endpoint? How long does it take the endpoint to see a USB message from a DAC and respond to it?
These are all important to SQ. If your latency is too high in some parts of the system, you will get stutters, pops, clicks, even wedging of the DAC (the bit-rate/bit-depth problem). So a faster CPU will reduce the processing latency in decompression and DSP. An SSD instead of a spinning disk will reduce the time it takes to get data off the disk (though modern OS caching will also reduce it even with spinning disks). Using a local disk instead of a NAS will eliminate network transmission time, and decrease the latency of reading a block. Using Ethernet instead of WiFi in congested or spread-out areas will reduce the latency between the core and the endpoint by reducing retransmits. The only good news here is that modern computers are so fast most of these issues don’t really matter. Network capability is the main Achilles heel here.
On the analog side, the output of the DAC, things are much more complicated because there are so many ways to get distortion. This is where you need a good electronics engineer with lots of experience. (Or a known-good pre-packaged circuit in this increasingly componentized world.) If you’re building with discrete components (resistors, capacitors, inductors), you’d think it would be easy. You pull up a circuit design program on your computer, plug some components together, simulate it, and see what kind of transfer function you’ve built. But in the real world, the components you’re building with aren’t perfect. They vary by as much as 20% in either direction. What’s more, components like capacitors are notorious for (a) changing their values over time (this is why you “burn in” your electronics), and (b) dying horrible and sudden deaths after a certain number of hours of operation (mainly electrolytics). Half the flat-screen TVs junked in this country are junked because of blown capacitors, and half of those can be fixed by replacing a couple of those capacitors. I’ve got some 45-year-old speakers I’m listening to right now; I haven’t opened them up to look at the crossovers, but I’m pretty sure they don’t sound the same way they did when they were new, because the capacitor values have surely changed. That’s why high-end capacitors in audiophile gear cost so much; they have lower variance, and don’t change as much over time, and often can handle higher and more abrupt current changes (for sudden and sharp noises in the audio which require the speaker diaphragms to be quite suddenly in some other place).
What’s more, there are physical effects which don’t show up in the circuit software. For example, if you’re building a crossover circuit for a speaker, you don’t want to physically align two of your inductors, or you’ll get inductive coupling which will introduce distortion. It’s a nightmare to get this all right. Well, a PITA, anyway.
Componentization to the rescue. I’ve got a little amp which is just a box around a board like this one with a TPA 3116 inside it. TI componentized the amplifier circuitry; someone else combined that with a filtered power supply. Two components.
Anyway, the takeaway for me here is, if you’re buying for the digital side of the world, think about latency, not electrical distortion, because unless you’ve got something like an bad floating-point unit or memory errors, the bits are not going to change, no matter how expensive or cheap your computer is. The key is whether you can get them where they need to go quickly enough. Spend your money on the analog side, where the circuits inside speakers and amplifiers really matter (though amplifiers are now increasingly componentized).