The Depth knob on both the Badonk and the Archetype is a control of the low frequencies in the negative feedback loop of the power amp. A simplified explanation is that it’s an active boost of the low frequencies between the preamp and the power amp. This is how a Presence knob works, but with the high frequencies. So you can think of the Depth control as a Presence knob for the low frequencies.
In addition, it un-dampens the low frequencies in the power amp and speaker interaction, which makes the low frequency a little less tame and controlled at the speaker. The whole power amp, speaker, negative feedback system can get pretty complicated. There is no need to go deep if you don’t want to. You can just use the simplified explanation above.
— Ben Adrian