But if the Amp already accepts speaker level inputs, there's no need for an extra high-to-low converter (which will add to your distortion level)... why not just tap into the speakers and feed the amp for the sub directly. Besides, most class D amplifiers used for Subs are not very good in THD to begin with (I don't think you can degrade it much), so SNR would be the bigger factor here, don't you think?