[archi42]

You're sure you're not hearing some harmonic? IIRC THD at lower frequency increases drastically. Or wind noise of the subs (they're moving lots of air)?

You can check with a measurement mic what's audible (& make sure to take measurements at multiple points in space due to the low wave length).

To decrease the lower frequency of the audio interface, you probably have to modify the output DC decoupling capacitors (those act as a high pass; bigger C = lower f).