Ive done what you said but the bass seems to be extremely high.
Question - is the volume on the sub controlled by the volume on your receiver? If so, then those are indeed preouts. If not, then they are fixed volume line level outputs - which isn't ideal, for sure.
I'm going to assume that they are preouts for the next part. If "extremely high" means loudness, you can control that with the "Bass Level" dial on the sub. You should be able to turn it way down, perhaps even to the point that you don't hear it at all. If "extremely high" means that it's playing higher frequencies (e.g. you can hear dialogue), you can control that using the "Crossover" dial. Above the frequency set on that dial, the volume will tail off pretty quickly as the frequencies get higher.