High-Level Inputs on Subwoofers

All our subwoofers come with Low-Level inputs only. 
RCA on the 1961 Series Subwoofers and XLR & RCA on the 1723 Subwoofers.

High-Level inputs on subwoofers have no audible benefits. You actually get a much better signal-to-noise ratio with Low-Level than with High-Level inputs, and in addition, the signal does not have to be converted down to work with the subwoofer's internal amplifier. 
You also do not have to worry about connection issues with your main amplifier topology, as connecting as a subwoofer via High-Level can cause problems and even damage.

For the very rare case that your amplifier does not have a subwoofer or pre-amp output, you can get a High-Level to Low-Level converter, and make the connection that way. Like this one here.