You can only wire up one impedance setting at a time. We will have little under-chassis upgrade boards with switches that will allow setting output impedance without having to disconnect anything, but that will still produce only one output impedance at a time. Yes, in general the various taps are for loads that are a the close match, so 4,8 and 16 are primarily for speakers and the oddball phones ( I think there are some IEMs) that are down in that range. For headphones from 32 ohms on up to 600 ohms the 32 ohm tap makes the most sense. I don't know if the K1000s would be driven adequately, but I would try the 32 ohm tap to find out.
