Indeed the signal path on both the preamp and amp is very simple. That said, the power supplies supporting it are rather complex and they are kept well away from it. Also, the preamp output transformer and amplifier interstage transformers took over a year of prototyping to get right. So the circuits are "deceptively" simple, but there is a lot supporting them, and as Lynn stated, these circuits are kept well away from the signal path, both electrically and physically.
Also, if you spend time reading this thread and the very long 300b lovers thread, you will see that the "deceptively simple" circuits are also cleverly designed to eliminate distortion at its source as well.