Is It Safe to Warm Up Power Amps Without Speakers Attached

It will damage tube amplifiers.

Worse yet is "moving over" speaker wires to the amp while its powered up. A good way to damage, possibly permanently, the output stage of the amp.

These statements are false.

What is true is that some amps are stable without a load and others are not.

Our (tube) amps are unconditionally stable regardless of load or input signal condition.

Most vintage tube amps are fine without a load. This is not always the case with modern tube amps. Just check with the manufacturer.

Hooking up speaker cables to the amp would not be a problem if the cables are not shorted. However, what about the inputs of the amp that is warming up ? The amp should be shut down, caps discharged before plugging into the inputs.

Worse yet is "moving over" speaker wires to the amp while its powered up. A good way to damage, possibly permanently, the output stage of the amp.

It will damage tube amplifiers.

It depends on the amp! A H/K Citation 2, Dynaco ST-70, no worries. Our stuff is no worries. Some amps (tube or transistor) can go into oscillation. Its best to check with the manual or manufacturer.

Always been told that a no no ...

For tube amps, pick up two 8 ohm resistors with at least a 5 watt rating. Quality doesn’t matter - only value and power dissipation.  Those rectangular, sand-cast resistors typically available at Radio Shack are fine for this purpose.

Put one across each binding post and you now have a friendly load presented to your amp.

The exact value doesn’t matter, but keep it in the 8 ohm range which will work irrespective of which tap your transformer is set to (4, 8, 16 ohms).

Cheers,
Thom @ Galibier Design

Some class D amps will go into oscillation without load

If it happens to be an Octave Audio amp, sure you can.

Be glad you asked.

+1 polk432  In actuality there are some tube amp designs that can deal with being powered up w/o a load, but most cannot.  So best not to try to find out.