@Miniperl a lot of back and forth for a simple question.

As mentioned, the speaker level input on the sub is high impedance, so will not affect the output of your amp. Some subs have speaker in and speaker out for convenience, but the output is not a requirement. The connections are in parallel, so effectively the speaker input and output are connected together. Likewise, the A + B speaker terminals on the amp are in parallel, so using the B terminals for the sub us fine.
Length of wire will not cause any phase issues, just keep the positive and negative terminals correct on each end. What will potentially cause phase issues is the placement of the sub so use the phase switch or phase control on the sub to adjust for that.