New control unit

As a simple plan I thought the 4 different colour LEDs could correspond to 4 different frequencies of sound. So red for the base, green for low mid-range, blue for high mid-range and white for above this.

This can be achieved using a low-pass, 2 band pass and a high-pass filter.

There are a few ways of doing this, active, passive or maybe using an ADC on a micro controller.

Active

This is basically filtering with amplification, but effective decoupling is provided if OpAmps are used.

Passive

This is very basic CR ladders, may be sufficient for our needs.

Analogue to digital

Could use the ADC input of a PIC to switch signal to a different port when a certain frequency threshold is reached? TBH this is my preferred solution as it involved the least amount of components and doing digital is sooo much easier even if the PIC is programmed in assembler - which is very cool, but I will probably use C for simplicity.

In all cases the 12V can be switched using transistors - old fashioned analogue things, but can be used as a switch so very very useful!