Recording on iPhone from stereo line-in

I'm looking for a way to perform relatively high quality stereo line-in recording on iPhone. Unusual and third-party hardware is acceptable, but jailbreak is not. I am aware of the following options:

Griffin iMic paired with Apple's Lightning to USB Camera Adapter

• Pros: Well-reviewed, USB interface.
• Cons: Lightning to USB Camera Adapter is not supported on iPhone.

Blue Mikey Digital

• Pros: Works with iPhone.
• Cons: Requires Lightning adapter, line-in quality reportedly poor.

Neither of those satisfies my requirements. Is there some workaround I'm missing?

Check out some of the electric-guitar-interfaces. For example, the Line 6 Sonic Port Channel Audio Interface has a stereo 1/8" input jack available. On the pricier side though.