Advise me on “dongle” type HiRes DAC/HP amp for iPad?

[MikePM]

A good place to start are the comparison charts for portable DAC/headphone amps at https://www.audiosciencereview.com/forum/index.php?threads/master-index-of-measurement-comparison-charts.8246/ 

 

While you may not think objectively good measuring equipment necessarily sounds good, I would argue that badly measuring equipment probably does sound bad and should be avoided. If the signal to noise ratio is at a level that it can't even fully resolve CD quality audio, it is not a good option.  The measurements at Audio Science Review (ASR) look at both the signal to noise resolution (SINAD) for the DAC and amp separately and it looks at the output measurements of the small amplifiers.  Its a good place to start and is helpful to at least eliminate some of the choices.