The module used to measure the effect of AC-coupling is an Intellijel Mixup.
Similar results can be obtained with any AC-coupled inputs.
30Hz square wave generated with a Klavis Twin Waves MKII oscillator :
Before and after AC-coupling :
Spectrum before AC-coupling (oscillator output) :
Spectrum after AC-coupling (Mixup output) :
30Hz wave generated with a Xaoc Odessa oscillator :
Spectrum before AC-coupling (oscillator output) :
Spectrum after AC-coupling (Mixup output) :
Audio captures
30 Hz square (50%) wave :
30 Hz saw wave :
30 Hz folded sin wave :
Resources
A very good explanation about AC coupling can be found in this Quora answer.