Fire pit dancing to music -- increasing the voltage from a subwoofer out
You want something basic, have a look below.







simulate this circuit &ndash; Schematic created using CircuitLab

That's just a block scheme I believe you already had in your mind, it's there only to help me tell you my ideas.

Let's start from the valve drivers. Of course you can have as many as you want. These little guys accept an input say from 0 to 5V or whatever is reasonable and scale it to the valve dynamics. They also adapt the impedance, i.e. that the box driving them will not need to source current: that's the driver's job. Inside these boxes there is an amplifier. I've just looked your valve spec, the current is not that high, you might get away with just an operational amplifier. Just look it up. The drivers may (and should) include some limiting (and safety!) devices to be extra sure to keep the output in the valve range and above all to be sure that if the system fails, it fails gracefully, i.e. you don't want the valve to be wide open. The driver will need some high voltage, somewhat high current power source (marked PWR).

Now to the bigger guy. You have only a line in input (you can hook it to the headphone output of any device) in addition to the power input (marked low voltage). The filter bank takes your signal, processes it and produces the voltages needed for the valve drivers. What happens inside the filter bank? You tell me. You may take a portion of the signal with a bandpass filter, reveal its envelope, multiply it for a fancy gain then send it to valve one. Valve two may be wide frequency driven, but only from signals above a certain treshold. Valve n may be driven by whatever, and so on. That's part of the specification. What is inside the filter bank? Well, somebody will eventually tell you. You might implement all the functionalities with analog filters and op amps, but especially if you want it iOSomething powered you better throw in a microcontroller, that will require an ADC to read the line in and some knobs and a DAC for the analog front end. That is the though part and nobody around here will tell you more than bandpass filter or envelope detector. That is huge, believe me. For the records, the filter bank is the thing with all the fancy knobs and lights on it.

Yes, filter bank is used in the broad sense of the word.