In reply to:

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It assumes that the amplifier's DAC is better than the player's, and I don't know that's the case.

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Unless of course you are interfacing with a switching amp with digital inputs straight to the digital circuitry, in which case there would be no DAC per se.

If the actual amp circuitry is completely analog internally, I would go for a separate DAC dedicated to the conversion process from digital to analog. Of course the quality of the DAC depends on many things including the DAC chip itself, power supply, component selection, I/V stage where applicable, board layout and analog filtering.

Not all DAC sound quality issues are measurable. Each DAC chip is said by many to have a sound signature of sorts, meaning that two DACs with identical specs may sound different. Some purists insist on using the older R2R DAC topology for most pleasing sound, even at the expense of lower performance than can economically be achieved with the latest delta sigma topologies. Using an external DAC will let you listen to the "flavor" of sound you prefer most.

Stu