Measuring amp output impedance

[mozz]

For transistor amps you are correct. Lower impedance may blow the output transistors. For tube amps it's the opposite. Tubes would rather see lower impedance than higher. If it gets to high or the speakers get disconnected, the output transformer will arc over. Many tubes amps will actually short the output jack to ground if you unplug the speakers.

[PRR]

No, "by ear" is more wrong than right.

There's got to be some sales-specs on any amp you buy. Hardly any amps are made in dark lofts or dense jungles anymore.

[Rob Strand]

There can be confusion about what "output impedance" means.
The question should be: How to determine the rated load impedance of an amplifier.  ie what ohm speakers ?
Output impedance can relate to damping factor. (The actual output impedance of the amplifier.)

There is a small relationship between the two in that a higher impedance
setting the amp will raise the output impedance and that's why you
can hear a difference.  *BUT* it's not possible to judge what is the
*correct* impedance setting on the amp.  Generally setting the amp to higher
impedances will sound a bit bassier.

For (output transformeless) solid state amps there's usually no impedance setting but
there is definitely a rated impedance.   If I have an amp rated at 8 ohm and I
load it with 4 ohm speakers (or 2 ohms for that matter) it will generally still work.   
What will happen is the amplifier will overheat with a strong likelyhood of frying the
power amp chip or output stage transistors, maybe even the PSU.

For a tube amp you can measure the transformer input and output voltages and deduce the
transformer turns ratio.   From the power supply and output tubes you can ball-park the
output power.  Then from that you can determine the impedance.  If the main PSU transformer
is underated you can still get a wrong answer.

You can also measure the transformer impedances.  Something like this.  (IIRC this video still had a crack/assumption in it.  I watched it a long time ago.)
https://www.youtube.com/watch?v=nNlPgirHP48
https://www.youtube.com/watch?v=5jUitplchok

[merlinb]

edit: delete