What is the lowest frequency that the W3 will pass? I want to make my subwoofer wireless and need to confirm they will transmit a low frequency signal, i.e. down to 20Hz.

The W3 will easily handle low frequencies and is designed to transfer all music without any compression of loss. If I have the W3 can I send audio to my passive rear channel surround speakers? Sure, as long as you have a power amplifier to power your passive speakers, the W3 will work.