I know you have good reason to sell, but I don't think it would cost that much to tap into the high (line) level outputs. You would just have to find an electronics whiz. $100 would seem fair. Likely under an hour of work.
I've never heard of a line level converter before. Obviously it would be best not to have that step, but I'd bet there are acceptable ones out there. Historically much of recorded audio has incredible steps to reduce noise and improve fidelity that has to be managed by the playing equipment. Think RIAA curves or Dolby equalization. Or even digital audio. So it is certainly possible to get a good result using a a variable line level from the head unit amp.
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