Experienced reviewer Skylab reviewed the PURE i-20 as a transport, which promptly caused him to sell his Wadia iPod transport. The notion of using the i-20 (with lossless material) or similar devices in a high-end speaker system is perfectly fine.
The built-in DAC did not impress Skylab so much. But here's the thing - you could start with it and add a standalone DAC later.
As to MacBook versus iPod/i-20 - I have a bias that built-for-one-purpose devices such as the latter pairing should sound better. But it's just a bias. I have not directly compared these situations.
Just thought I'd add my 2cents - comparing Tidal Hifi streaming from
iPhone 5s->adapter->Pure dock->No brand Coax cable->Auralic Vega DAC versus
iPhone 5s->adapter->Pure dock->No brand TOSlink->Auralic Vega DAC versus
Macbook->Lightspeed USB cable->Auralic Vega DAC
Listening to Hey Now, London Grammar, the vocals and base line were similar with the Pure dock. The Coax had more air and space via the Coax, but it was very close.
However, the sound quality from the Macbook and USB into the DAC was significantly better than via the dock. More detail, space and sound stage.
This surprised me, I would have thought the two would be closer.
I guess I'd put this down to the electronics in the dock that's stripping the digital stream from the phone.
Guess I'll be keeping the Mac around for a bit longer.
(Music via Tidal was also not surprisingly significantly better than Apple Music and Spotify. I suspect that there would be less audible difference on lower bit rate stream between the dock and Mac)