If you need to run 350watts to a sub stage, you could get away with an efficient 5-channel Class D amplifier, such as Alpine MRX-V70 or Polk D5000.5. Most of the time, they won't need to see even 50amps of current. Class D amplification is fairly efficient, and don't need that much current to play loud. A lot of alternators give you easily +100amps of current. Even with slower pulleys, what you lose due to slower pulley speed is not a big deal. Don't also forget the batteries. They're a great and still the most important source of current you have.