FWIW in car audio people have used what is know as a line level booster but not sure what is available for home use or how much distortion may get introduced. Keep in mind that car amps also have adjustable gain control which would be nice to have on home gear for these situations. Also most if not all car audio decks use 5v output for noise rejection purposes. I still see some at 2v but not very often. Automotive DC electrical systems are pretty noisy . You can check your pre out voltage with a standard dvom meter just search on youtube. Bear in mind you will get max voltage but you will be unable to tell when distortion is introduced.
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