Quality answer to dangers of Tube amps running under no load

I don't really agree with that explanation, it applies more to an ideal transformer. If there is no load on the secondary there is no magnetic field from the secondary current to reflect so the primary becomes one big inductor. An inductor's impedance is frequency dependent, so when the power tube's current hits the higher impedance the anode to anode voltage surges. The higher surge voltage does the damage.

A more immediate threat is if the speaker cable is suddenly disconnected with a current through the secondary. The magnetic field collapses and the stored energy in the primary and secondary has nowhere to go and causes arcing from the induced voltages. Same as a car's ignition coil operation, only the collapsed magnetic field has a relief path across the spark gap. The tube amp OPT has no such safety valve unless you put a few diodes in series with the B+ to ground. But the problem with that is diodes tend to short and that would cause a short through the transformer. 

As a precaution, what I do with spade terminal speaker cables is mount them facing down on the amplifier binding post so if they should become loose the weight of the cable won't  pull it off the post. 

I can tell you this.  Cary Audio burns in all the tube amps on several racks and powers them on for 3-5 days with no load.   They are biased and left to burn in for days without any load on them.  

@cavscout19d I appreciate you sharing the detailed explanation of the ‘how and why’ tube amps need to see a load upon power-up. I found out the hard way when a blew up a beautiful old Crosby radio because I didn’t know the speaker needed to be wired up. I accept @gs5556 correction concerning ‘ideal’ theory vs. actual implementation. I would be very curious to learn how Cary builds their tube amps so that speakers don’t need to be connected. I did run across a video recently in which a tube amplifier incorporated a very special RCA jack that I think was shorted until the RCA plug was inserted. It was in a video in which XrayTonyB explained how ‘multiplex FM’ worked. The video featured an H. H. Scott FM de-multiplexer that de-coded the FM Stereo signal so that a mono FM tuner could feed a stereo amplifier, but I digress. 

I get a lot of amps back that customers powered up forgetting to hook up speakers or a load. Usually due to excitement. Depending on the topology, the damage can be different. In this situation, the cathode resistor goes up in flames. 
I hot swap speakers all the time in push-pull amps which has never been an issue as it’s only a few seconds. 
i recommend not powering up a tube amp without a load. Not worth the risk. If need be, grab a couple of 8 ohm 50 watt resistors to have around. 

Solid state amps can also do this.  My Sunfire Signature sub is now not working because it powered up without a speaker connected.  The amp had been separated from the speaker cabinet.

My "Universal Switcher" unit (for use of comparing any two components/speaker systems) has provisions to switch in 261 ohm, 30 watt resistors for use with tube/whatever gear so that even when the relays are open (milliseconds) during switching operations, there is a load on the amp.