You will get a cleaner signal (bass) from the line level, as an amps speaker output, is far dirtier.
Also this is what happens inside a sub, follow the speaker level input and it goes through a "dropping opamp" IC11A and then goes to the "line level rca input opamp" IC11B then to the rest of the subs amps, so by using line level you also use one less opamp as well. https://ibb.co/3cmCWC8
Cheers George