Re: head units and clipping
Yes, the speaker outputs generally will be heavily distorted and clipping well before full volume. RCA outputs are designed to reach full voltage on a logarithmic scale, with max voltage coinciding with max volume. Generally, you want to set up your amplifier gains so you're using almost all of the volume range on the head unit. If you're using a mix of speaker level outputs and RCA outputs you have to keep the volume lower. The 75% rule is just a guideline for people who don't have an oscilloscope to check the pre-outs for clipping.
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