Eyeing on my next build, I measured a smartphone pgm output to be completely muted with the channel potentiometer at 57,3kOhm.. would a 10kOhm summing resistor (series) and a 50kOhm dual log pot do the trick?
I haven't measured with a CD player but I suspect they have higher output (a CDJ-800 overloaded the preamp ins of my first build).. use a 25kOhm trim perhaps?
EDIT : there's the EQ circuitry in the signal path which I didn't take into account.. I measured the HF series caps as having 290R resistance (the LF has 220R resistors) with a multimeter, is this the correct way to measure them? Iirc parallel resistance is calculated as 1/R1 + 1/R2 etc.. or ape the EQ circuitry in the new build to compensate (the input to output resistance was ~57,4kOhm)