Amplifier used w/ headphones and no speakers connected, can it be damaged?

You can easily test this by leaving the speakers hooked up and plugging in the headphones.  The speakers should cut out with the headphones plugged in.  If that's the case there's no issue at all.  

And if you aren't going to use your Harbeths anymore, send them to me. ;-)

@ken6217 

I bet that sounds glorious. I didn’t know you could do that. High powered tube amplification for headphones are one of the most amazing things I discovered. I us a 300B Woo WA5… that can be used as an integrated amp. Took my headphone system to a new level… after over 40 years or reaching for the best possible headphone sound.

I had seen other people on the Abyss forum use speaker amps fpr thei TC and so I wanted to give it a try. I first used a Simaudio 600i V2, and then decided to try the AR amp. iI sounds great and there is no hum or noise through the headphones.

I did have the WA5 many years ago.

Class D amps it is no problem. Class A and class A/B it is almost always bad.

Class of amp has nothing to do with it.

as many have stated. Solid state amp not a problem (unless it happens to have an output transformer.. most do not. .)

Tube amps can be because of the output transformer. In normal operation, the energy that the amp output tubes send to the transformer is absorbed by the speaker. If the speaker isn’t there, that energy is reflected back to the output tubes and can cause problems.

also as stated, if in doubt, contact the manufacturer because somebody here will almost always give you bad advice... (maybe this is 

Short answer is no, it won’t damage the amp.  Headphones are supplied signal from a different source inside the amp.  Since you have no speakers connected, there is no load on the main amp.  I’m surprised you don’t have a speaker on-off control?