But there isn't a traditional stereo signal in there - there's a mono signal fed to a crossover which splits the high/low audio and feeds it in to the amp as 2

But there isn't a traditional stereo signal in there - there's a mono signal fed to a crossover which splits the high/low audio and feeds it in to the amp as 2 separate channels. The (normally stereo) amp is then used to feed the tweeter and woofer instead of L/R.