I should have done this earlier in my attempt to understand the bandwidth of APT signals, but for some reason I didn't think I'd find anything. Anyway, I've now searched Wikipedia in a quest to furthering my understanding of the bandwidth requirements of weather satellite APT transmissions.
Wikipedia had a great description of both what is required, why and the effect of insufficient bandwidth. The specific details are here:
Basically, the APT transmissions are apparently 34kHz wide, and if the receiver's bandwidth is too narrow then you can expect over saturation of the lights and darks – exactly what I've been trying to improve the last couple of days (thinking it was an issue with audio level).