I am not an expert on input topology, Technic and VP=E are; however the answer you received doesn't sound on point. It sounds like they read your query on whether or not it accepts HIGH level inputs.
The balanced-diff input IS a LOW (Line) level input. Alpine Tech support may be assuming you are trying to use speaker inputs.
Again, I am not the expert on this, but PDX is a popular amp choice in this car.
FS:Sub Amp - works w/ Technic harness! MRD-M300 - cheap
DON'T BUY FROM RAF555!!!! Promised fix! - strung me along til Paypal dispute period expired!