It has been my understanding that no load is OK with solid state and a big no no with a tube amp. I'm sure there are exceptions. A lot of SS amps have headphone jacks that silence the speaker.
even there, one should be careful about drawing conclusions about what's going on 'inside the box', as that can lead to questionable user mods or practices.
unless quite low powered, it is very unlikely that these SS amps actually disconnect the speaker and feed the amp's power output to the headphone jack. instead the jacking-in of cans probably redirects an intermediate amp signal away from driving the power (output) stage and over to the 'phones. speaker and power amp remain safely, blissfully and silently in union. however, some might interpret the speaker's silence as evidence of "disconnecting the speaker" and decide it's okay for any of various purposes to run the amp with a disconnected speaker/cab. so for a SS amp they came to a correct conclusion for the wrong reason. where the problems are: it is entirely possible to use a similar signal architecture (removing signal drive to input of power amp) for creating a 'safe' headphone out in a tube amp (power amp and speaker remain always connected). but there, drawing the that same erroneous conclusion that the speaker is being -- and could be -- safely disconnected in a user mod could lead to mucho nastivity.
-=tension & release=-
