R. Edmund Lord marketed a sub bass speaker to accommodate people using receivers lacking preamplifier outputs and / or small low frequency restricted speakers. Along with RCA inputs he included speaker / high level inputs as do many manufactures.
Using the more widely used preamplifier outputs directly to the subs low level inputs bypasses any noise, frequency restrictions and distortion imposed by the speaker amplifier and its input and output stages. As with any sub you determine any audible advantage.
Monoprice or Blue Jeans cable offer economical custom length RCA or XLR terminated interconnects.