I've dones the searches and have gotten some spotty info about this. My questions are:

Does an aftermarket H/U hook into the RF amp via pre-amp or speaker level outputs? Is an adapter needed?

If speaker level, is there a limit on how high-level the speaker outputs should be? The H/U's I'm considering are the Clarion ADB340MP, the Kenwood DPX-MP4070, and the JVC KW-XC77. These are all double DIN, CD/cassette combos with AUX input. Other recommendations with those features would be welcomed.

Thanks in advance,
Joey.