In Dreams I Walk With You wrote:

I checked the manual and it says that when setting "small" is chosen then
all freq's below 100Hz will be passed to the sub. So I can assume the
crossover freq of the amp is 100Hz ? So then it would be best to set the
crossover freq of the sub at 100Hz, correct ? Thanks again,

Your sub doesn't need a crossover if the AV amp is providing that function.
Set the sub's crossover to off or its highest available frequency.

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Eiron.