Couple of things
If you're trying to have lights on if the bass speaker is enabled, then you can use a double pole single throw relay, this relay actually has two sets of contacts in it, so one of them will power your lights while the other one passes audio through to the speaker. The only problem I can anticipate with this is that your speaker might pop/boom when connect/disconnect it from the amplifier using the relay - this is something you'd have to just build and try it. But this would be very easy to build/add to your audio set up, it would cost ~$10 buying parts from Radioshack
If you want lights that react to bass and light/dim as the sound intensity varies than you'll need a more sophisticated controller. There are several ways of doing this, they vary from using a microphone to detect noise intensity to a circuit directly connected to the audio headed to the speaker. There are circuits available online (and products you could just buy that would also work)
If you could specify which way you're looking to go we could probably help further.