Just and FYI, they do this for systems that do not have a mono sub pre-out, AKA stereo setups which actually is intended to give you more compatibility. The sub is mono, so regardless of which input you use or if you use both even with a stereo signal, the signal is still summed to mono. In addition, most subs will accept a single input into one or the other. Sometimes the manufacturer may identify one as the preferred input, but if it isn't then it doesn't matter which one you use.
