It would appear to me that the option for using the high level inputs to connect the sub to amp outputs was included for the case where the sub was used in a two-channel system that lacked any sort of low level subwoofer output. Think of a case such as a "traditional" stereo pre-amp - there is typically no bass management in the pre, and the assumption is that the speakers used will be as close to full range as possible, making it impossible to connect a subwoofer without either intercepting the left and right channel pre-amp outputs or somehow tying into the high level outputs at the amp. In your case, both the AVM30 and MC1 have built-in bass management and a low level subwoofer output, allowing you to use the hook-up instructions on pages 16 and 17 - which would actually be the preferable approach, since it allows you to make use of that bass management.