I was just thinking about this. I pondered what way to use, and what way would yield the least amount of draw from the amps, and also not altering set resistances.
I was thinking about using 3 good old fashioned diodes wired reversed bias to give the set .7ish volts multiply by 3 and the voltage would go to 2.1V, I later figured this way won't work at high volumes even with high rated diodes, the peaks and valleys of the audio signal will saturate it constantly.
Then I though caps and resistors, set size of the cap to the given frequency range and voltage through put at max peak, using like a high pass filter even though it sounds like it won't work but the signal source is set to a certain crossover like low pass. Using a high pass cap (lower value uF), the peaks of the low bass hit still yield practical amounts of voltage and using resistors to limit current and a zener to limit transient voltage spikes. The negative to this is propagation delay, and the RC constant.
Voltage divider signal source LED driver, Using relativly high resistances to divide the voltage equally, send the hot lead (signal source) thru a in-line cap/resistor coupling to the voltage divider node, this divder node is wired into a Transistor base. This is just the set up idea, acting like a high power signal coupled to low level signal amplification that would use a 12VCC and a Transistor wired to go no higher (peak) that LED limits with current limiter resistor, and clip as soon as it hit zero crossing.
Remember these are just ideas rolling off my head, so don't lecture me. These are just ideas for 1111 and well myself and anyone who cares to read.