[Doug Freyburger]

Dick Pierce wrote:
ChrisCoaster wrote:

It's the same wherever I go. In the car, I switch from my mp3 jack or
CD to a FM station and instinctively I have to turn the volume down by
at least 1/3rd.

There is of course the alternative explanation that the
average signal level generated by your MP3 device is less
than a perfect match for the expected input level of
your auxiliary input, and that it's a simple gain mismatch.
If your MP3 device shows a consistent lower level on several
playback systems, that could well be the case.

Exactly. I remember that happening when I switched from a cassette deck
to a phonograph both connected to the same power amplifier decades ago.
Cassette decks usually had gain knobs and I would have to turn them way
down to even slightly match the level of vinyl record output.

If the MP3 jack is the headphone one it makes sense that this would
happen. If the MP3 jack is USB it makes no sense other than the
manufacturers didn't bother to agree on typical gain levels.