I have a Denon integrated amp which has 'pre out' connections. On studying the circuit diagram I found that these actually come from the headphone output circuit vis a resistor network (basically a speaker to line level converter as described above). My first thought was - Why would anyone do that? Then I realised that the amps input sensitivity for general inputs is 150mV and these are basically sent straight to the input of the power amp section. The standard true preamp/power amp levels are around 1V. This arrangement saves them from needing to add a line buffer for the pre out connection.
But the point is that it seems to work O.K.
But the point is that it seems to work O.K.