Inputs on the Subwoofer are a completely different subject. We were talking about Outputs on the Sub.
The Inputs on a Sub come in two types. One is the standard RCA-Phono Type (red, white) connectors, these are called Line Level Inputs, and are made to connect to amp Sub Outs or amp Pre Outs.
The Speaker Terminal Inputs on a Sub are referred to as High Level Inputs, or Speaker Level Inputs. If your amp does not have some type of Sub out or Pre-amp out, then you can connect the Speaker/High Level Inputs directly to the Speaker terminals of the amp. Internal to the Sub, those high level speaker signals are attenuated down to a level that is compatible with the Line Level Inputs of the Sub.
It simply give you two different ways to connect your amp to your Subwoofer.
The Speaker Terminal outputs on most Sub have a filter to stop low bass from getting to your front speakers. That leaves the Sub to handle all the low bass and eases the strain on the main front speakers. However, as I have pointed out the filter method is pretty crude and unpredictable, and would really only be needed with very small limited range front speaker.
The reason I asked about your amp is because I was wondering if it was a Stereo amp or if it might be a Surround Sound amp. Cambridge does or did make nice Surround Sound amps. If a stereo amp then everything we've said is true. However, if it is a Surround Sound amp, then likely it has some type of pre-amp or Subwoofer outputs, and that is what you should be using.
Again, we assume you are using a stereo amp, and if you are, and the Stereo amp has Pre-Amp out, then that is what you should be using. Though the Speaker Level Inputs on the Sub are fine, and if it is more convenient for you to use those, don't worry about it.
Steve/bluwizard