The problem you were having was most likely some parasitic oscillation. Very often amps that don't have grid stoppers can suffer from this. The crackle was probably spiking voltages due to a positive feedback loop between the amps output and some earlier stage. This positive feedback loop isn't a part of the circuit. It's caused by radiant fields inside the amp. Layout and grounding in ambitious builds (such as a stereo amp) becomes ultra critical to avoid problems with component and lead proximity. Search the forum and you'll learn more than you ever wanted to know about grid resistors.