When buying an old box,often times the line-in will need cleaning.Off the top of my head,among the most notably sensitive in my collection would be my G.E. 3-6000.It has the much preferred dedicated switch for auxiliary.This box has loads of bass and excellent loud highs.I love its retro-mod layout,simple controls,and solid build.With a sensitive line in,beautifully sleek looks,and great sound its one of my favorites.
One of the least sensitive I've experienced would have to be the Magnavox D8443.Its line in is VERY weak,almost inaudible,and requires engaging the tapes "record" function (clean heads).Strangely if you input through its line out,it is slightly louder.I've tried putting 2 headphone amps in a series between the source and line out with only moderately loud results.
My D8300 with its similar speaker layout is a much better box than the D8443 in every way (except looks).The line input is very sensitive although it requires engaging the tape "play" (clean heads) to hear anything :annoyed: . Its also better sounding due to its equalizer.I only wish they had made it better looking.