Well, it’s possible that a $35 DAC generating analog signals may not be comparable in performance to, say, a Benchmark DAC 1. Analog signal generation needs a lot of care in power supplies, circuit layout, etc. But if you’re using a Hifiberry Digi+ series card instead of an actual DAC, and then feeding the signal to a good outboard DAC, it’s likely your $35 (or $70 or whatever) media player will be comparable to any $2,000 player. It’s not difficult to get a reasonably steady stream of digital bits out — the downstream equipment can do the difficult job of converting it to clean analog signals.
I am not implying that all DAC daughterboards for the Raspberry Pi are of low quality. I would tend to believe that the ES9018-based DAC daughterboard from www.Audiophonics.fr, a cool French company, is of a quality high enough to compare against most high-end standalone DACs. Interestingly, they recommend the use of an outboard linear power supply, not the switched-PSU wall wart which comes with Raspberry Pi by default.