The 15 kHz passive LPF works fairly well, but has phase shifts in the treble end, which could mess a little bit with the stereo imaging, but it should be acceptable. It needs to be fed from a fairly low impedance source, so it would be worth adding an op-amp unity gain buffer: If you use a low noise dual op-amp (TL072, NE5532, LM833, 4558 will all work) and obviously duplicate the circuit for stereo, it will buffer your filter from the outside world.