I ended up turning a guitar amp up to ~100 decibels and recorded a take using my attenuator, and then a take using the in-app gain control. I set both the attenuator and in-app gain control such that the recording level peaked around -12 db.
Looking at the wave forms, I believe I ended up with brickwalling using the in-app gain control (Take 2), while the attenuator looks and sounds to have recorded more cleaner audio (Take 1).
My setup right now is MS-EHB-2 -> SP-SPSB-10 -> SP-ATEN-1 -> MetaRecord iPhone app.
Thanks for everyone’s feedback, I’m just getting started here and appreciate the help!