Here's the final circuit if anyone's interested in building a passive volume control using 2k potentiometer or a 2k stepped attenuator. The previous post ^ shows the values of the resistors to be soldered between each pin on the stepped attenuator. 2.2 ohm, 3.3 ohm, 4.7 ohm, 4.7 ohm, 6.8 ohm, etc...) They're in the column at the end. The Yellow highlighted part shows the actual resistance values at each switch position when combined with the 820 ohm resistors to ground, which in parallel reduces the overall resistance.
NOTE: This stepped attenuator circuit is the type that has constant input impedance, unlike others where the input impedance changes with each volume step.
The main switches are 4P3T (4-pole 3-toggle) so it can support two balanced audio channels. I'm using Rotary switches from Maplin.
It has a "Mono" switch (DPDT) which electrically sums both the L and R non-inverted (hot) and inverted (cold) signals together.
I may refine the circuit in future so it is almost exactly 600 ohm input impedance, but I'm happy with this in its current form. Most audio interfaces output at <200 ohms anyway, so it should be fine for most setups.