The cheapest 5-channel amplifier I'd consider is a PPI BK580.5. MB Quart ONX4.80/500 would also be acceptable. The Boston Acoustics GTA 5-channel is about on the same level as MB Quart. The Boston Acoustics GT series were considered pretty high end, but they did cost a lot more. The only issue with all these is that being class A/B on full range channels (PPI is A/B on all channels), they're quite big and need good air circulation on a hot day. A Class D amplifier could be more compact and run cooler. They do cost more, but not by much. A PPI P900.5, Alpine MRX, or Polk D5000 cost about $200-300.