Treebeard1, regarding your question about output impedance, it was said earlier in the thread that the three pairs of outputs of the MZ2 (left and right preamp outputs, speaker outputs, and headphone outputs) are wired in parallel for each channel, and the same signal is provided to each of the three corresponding connectors. So the output impedance is 2 ohms regardless of whether the MZ2 is used as a preamp, as a 1 watt (into 4 ohms) power amp, or as a headphone amp.
And it will have no problem driving your Halo A21.
Regards,
-- Al
And it will have no problem driving your Halo A21.
Regards,
-- Al