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There are a few more blanks.

a. it has to be a dedicated Mac Mini for music playback not something you use to file taxes, surf the net etc as well. That means disabling as much background processes and optimizing the system for audio playback

b. keep the external storage on a separate bus from the SPDIF connection (FW and USB or vv respectively) and use a good USB-SPDIF or FW-SPDIF connection (like a Wavelink HS or Offramp)

c. run Audirvana Plus

If you keep the DAC to 1-1.5k, I am going to say 10 years

If you don't limit the price of the DAC, I think the sky's the limit.

I think with A+ and the USB-X box on my Playback EMM Labs, the difference becomes very very close.

Have a look at the Mytek DSD. It does native DSD decoding and has been very highly rated.

At 2k, Benchmark is releasing their new Benchmark 2 which also does DSD decoding and comes with 2 analog inputs (preamp capability).