Some Head Units will allow you to turn off the internal amplifier. In theory this will eliminate any noise from being introduced into the pre-outs. This is more a marketing issue than quality of sound. The specs rated for your THD, pre-out ohms, and signal to noise ratio are all with the amplifier in use with the line outs. Don't worry about using. The only issue you may have is added heat being made from the head unit. A tight fit on the radio and clustered wires can reduce the heat distribution and promote heat failure. Still, this is more a install issue than product.
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 2003 Maroon Alero GLS Sedan. (Purchased new in 2003 with 18 miles, and sold in 2006)
2004 Toyota Corolla LE (Purchased new in 2003 with 87 original miles)
2007 Toyota Camry LE (12 original miles)
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