Probably just during transient peaks. I had a set of DCM speakers (I think) that had a feature like this. The bulb was visible (a "gee whiz" thing that helped sell speakers, I am sure) and lit once in a while. I guess it just transforms what could be damaging spikes to light energy by charging that filament. Dunno... surely the US-spec Bose have the same thing (?). Did you check their site?

Edit -- *I* just checked the US site and they're not mentioned. Just that that speaker has "protection circuitry." Presumably the same light bulb.

__________________
1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)