Is It Safe to Warm Up Power Amps Without Speakers Attached

A friend brings his amp over to try with my speakers. We listen to my amps first. He plugs in his amp and powers it on to warm up for an hour before we power down my amps and move the speaker cables to his amp. Is this potentially harmful to his amp? I recall reading in a manual or an online post that someone suggested never running an amp without a speaker load. Does it depend on ss vs. tube...SET or push-pull...etc.? Cheers,
+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. 
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).

Thom @ Galibier Design
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.
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.
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.
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.