I was surprised to learn that the Apple Lightning-3.5mm dongle contains a rudimentary DAC and amp so I was looking for an alternative this week in Hong Kong to use with Shure SE846 while I'm travelling. Some basic Googling suggested the Fiio i1 would do the job although I was put off by all the extra cable. I found a bunch of small specialist headphone shops in Sim City where Mingo Headphone (
http://mingo-hmw.com/new/) suggested the db Magix AC3. In fairness he didn't have the i3 but the selection of esoteric cables and IEMs suggested the AC3 wasn't stocked by chance.
To answer the impedance query - the SoundWise app for the iPhone allows you to update the firmware on the AC3, and turn on 'Hi-Fi' mode to boost the output. With 'Hi-Fi' turned off so it's in standard mode then the Apple dongle is louder by two-clicks of volume (hardly an accurate measure, I know!)
Does it improve the sound quality? Yes, absolutely. It's a vast improvement in clarity which makes the Apple one sound cluttered and muddy. Soundstage is clear and involving. Build quality feels nice. So far I have no complaints and it seems worth the money.
Just found the spec's on their website:
Frequency Response: 20Hz-20KHz
Maximum Output: 30mW @ 32Ω (Hi-Fi)
Drive Ability: 16Ω - 60Ω
Dynamic Range: 122 dB
SNR: 122 dB
THD+N: 0.0004%
Plug Type: Lightning Connector
Weight: 15.0g
Dimensions: 39.8mm x 43.8mm x 10.1mm