Never heard one in action, but the Firestone components are generally highly regarded and parts quality is usually pretty solid, if not extravagant.
In my experience Apple components DO benefit audibly from a DAC upgrade, with the caveat that the amp/speakers are revealing and of suitably high quality.
If you've got a good price, I'd go for it. The DAC is, in my opinion, the most useful/versatile thing ever invented as it (potentially) improves the sound of several components.
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