here is how I understand the I/O block diagram for measuring loudspeaker impedance based on your video tutorial.
I'm using the following equipment;
iPod touch 5G with 30pin to lightning adapter
Line 6 Mobile In
one observation with this setup; I'm not seeing the option for (Ch2-Ch1) input, just Ch1 and Ch2 independently.
Would using a power amplifier be advisable to take some current load off the iPod/iPhone/iPad headphone output ?
Are you aware of any iPhone/iPod/iPad compatible USB input devices that have floating inputs ?