I actually think I can handle that. I have an email into Art Audio UK, I'll first see what they say.Hi Mark,
If they can't provide the information for your specific amplifier, ask them what the damping factor is for the standard configuration that is optimized for an 8 ohm speaker.
The output impedance of an 8 ohm tap equals 8 ohms divided by that damping factor number. You could compare that result with the output impedance of your specific amplifier, as determined via the measurements I described.
I worked through some equations, and the output impedance of your amplifier can be calculated from those measurements as follows:
Let V8 represent the test tone voltage measured into an 8 ohm resistor (connected to the 8 ohm tap, if there are multiple taps).
Let V4 represent the test tone voltage measured into a 4 ohm resistor (connected to the same 8 ohm tap, not the 4 ohm tap, if there are multiple taps). The volume control setting should of course be the same as for the previous measurement.
Let R represent the amplifier's output impedance.
R = (8(V8 - V4))/(2V4 - V8)
I suspect that the answer will be around 1 or 2 ohms for the PX-25.
Also, for several reasons the test should be performed at a low volume level, as monitored with a speaker connected to the channel you are not measuring. Given the 101 db efficiency of your speakers, at low volumes you'll probably be measuring voltages that are in the rough ballpark of 20 or 30 millivolts or so.