I would use an active line output converter like an Audio Control LC6 or Cashe 2 channel or 6 channel LOC. Passive converters are cheaper but the fidelity is worse especially on the bass and the higher you turn the volume on the deck, the worse the bass gets. Active LOC's also ahev line drivers to increase the signal into the amp. You could also incorporate a JL Audio CL-RLC if you wanted to have a remote bass volume.
Some amps, like JL Audio, allow you to connect speaker wire to an rca end and plug it directly into the amp with no LOC needed.