The built-in sub amp only has to power the low-end frequencies, and is voiced for that specific speaker. It doesn't have to perform across the entire frequency spectrum as your power amp does.
As a result, your amp now has less of a load to drive. Your amp won’t have to deal with low impedance demands.
As a result, your amp now has less of a load to drive. Your amp won’t have to deal with low impedance demands.