is it ok to leave a tube amp on without speakers connected

So it's a good test. No input signal, no speaker load, if it gets damaged - it's a junk amp.

Dynaco's ST70 manual does not recommend leaving the speakers off.

The trick is make sure there is no signal input.

For modern amps best to check with the manufacturer.

Maybe this one worth reading

Like others report: My research indicates the voltage builds up and damages the output transformer. I wanted to disconnect one speaker so I could do the Soundoctor.com trick to integrate my sub. Was told never run tube amp without load (by Upscale guys) but you can use a 10 ohm 10 watt resistor in place of the speaker https://amzn.com/B0087ZD6FQ. I think they do that when they are working on an amp and connecting a speaker is inconvenient. Decided for my purpose I could just disconnect the input from one amp channel.

What does the manual/manufacturer suggest or warn against or not. It's a yes and no answer. My Cary Rocket 88R  does not need to see a load and will not damage the amp by design.  Others will require a load to be safely used and not damage the output trannies.  Never follow the advice of anyone other than the manufacturer of YOUR product .