Andrew -- that's a neat observation.
I just realized -- the light bulb doesn't limit the power output to the same value as the bulb wattage; it effectively functions as a clipping detector/limiter right?
As the audio signal gets more compressed and shifted due to clipping, the light bulb will glow more, and it's resistance will go up.
The corollary to this is that fast, high-energy transients will get let through without any significant resistance at all. Quite suitable for music!
Do I understand correctly?
For what it's worth, I searched my memory and I'm pretty sure that the bulbs used in the speakers I'm thinking of (HUGE speakers, two filled a 2,000 sqft room to very loud levels) were 1156s. IIRC these are 23W 12V automotive bulbs.
For the home designer, the 1157 socket style is probably a better [single] choice, because you can use dual-filament bulbs, providing a much wider selection of wattages. As you pointed out, there are 5W bulbs available in this form factor, but I can't remember the p/n... 2397, maybe?
Wes