I would say that by far the most significant factor is psychological. For most people, there has to be an element of 'expensive=best', or 'physically solid engineering=good sound'. And you can get hung up in loads of other sub-categories like 'discrete circuits sound more open than ICs', or 'surface mount components suck the life out of music'.
The objective tests can be a mental crutch, or they can be a source of constant doubt ("The $50 DAC measures
almost as good as the $10,000 brand. What exactly does that 'almost' represent? Maybe I've got to just bite the bullet...")
And cables...! You may get into the mindset where you want to believe the arch-pragmatists who tell you that all cables sound the same. But then the doubts set in and you begin to think that they are arch-pragmatists and therefore probably a bit philistinic in their tastes; maybe they're just not capable of hearing the finer aspects... And also, many of these arch-pragmatists seem to use cables that are more expensive than they could be, almost as though they don't really have the courage of the convictions they are promoting to everyone else... You need to get into the mindset where you *know* that cables are making no practical difference whatsoever, but you are confident that they are solidly engineered. e.g. something like Belkin springs to mind as a reasonable brand.
You need to get to the place mentally where the only thing worth worrying about is the speakers:
- solid, powerful, class AB amplifier - maybe even an AV amp for ultimate value for money and peace of mind vis-a-vis safety, remote control, robustness, shutdown in case of short circuits etc. As we have seen with headphone amps that emit smoke, damage headphones etc., amplifiers designed by 'industry mavericks' are fragile, unstable and dangerous. AV amps are also where you're going to get a beautiful brushed aluminium enclosure for a couple of hundred dollars. Unfortunately I don't know enough about HDMI and the fact that the amp might have a built-in DAC. I'll bet it's fine, but I'm sure I saw things regarding jitter and HDMI...
- Any DAC that measures reasonably well and is unlikely to blow the speakers up if the power goes off or a USB cable comes out, etc. Many cheap USB-powered DACs are going to have that problem and no one but me seems interested in it - from practical experience. There is no need to fear S/PDIF or optical equivalent as the interface and it may just be the most professional, stable way of doing this. (Asynchronous USB or ethernet may be the theoretical best interface, but not worth it if it leads to a fragile system that breaks when the OS updates, etc. - assuming your system is PC-based).
By far the most important thing is to get into the mindset of knowing that the audio signal isn't super-fragile in the sense that a tiny amount of theoretical jitter (S/PDIF might lead to some jitter that's unmeasurable but theoretically there) isn't even on the radar as regards changing the sound. Ditto differences between cables. Ditto any reasonable amplifier's specs. Ditto any reasonable DAC. Ditto any reasonable volume control.
Save all your energies and money for the speakers.
If I wasn't into active crossovers etc. my ultimate affordable system would comprise Quad amplifiers and a pair of KEF105/2 equivalents if only they still existed. As regards the DAC I would even be happy to drive it from the headphone output of an iPad.