I have the PDX-5, but didn't pay $600, rather $350 online. Do a search for it and you won't pay that much. Here's an amazon link, but there are even cheaper options with shipping paid for.
Amazon.com: Alpine PDX-5 - Amplifier - 4-channel plus Mono-power: Automotive
Also, mine came with the "birth certificate" noting it was tested at 108 per channel for the speakers and 412 for the sub channel. Its nothing to sniff at.