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easy to google, high level input,

https://www.upscaleaudio.com/pages/subwoofer-tips

Makes sense, I just read that some sort of high level input into a sub amp, if available, can be used straight from main amp output, never saw this in action though. Wasn't sure if the BX63 could do...

First of all I don't run my BX63A crossover for the B460 subwoofer like the manual says. I tap off a pre to amp interconnect to run signal to the BX63A. But I don't use the main out of the BX63A back...