First define 'neutral'! If you are looking for an audiophile grade solution, I'd consider a pair of R-2R multiplying DACs built from gold plated contact reed relays and precision non-inductive resistors, controlled by a MCU that returns to a clock stopped deep sleep mode (to avoid it contributing any digital noise) once it has made any attenuation change until it is woken up again by further user input. For audiophools,ditch the MCU and control the relays from pair of concentric rotary switches via a diode matrix that 'programs' the attenuation steps. For the rest of us, the best digitally controlled volume/balance chip we can afford is probably a better choice!