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

Definitely bad with tube amps,  however some solid state amps aren't bothered by it.  Send Sugden an email.   

I have a dedicated (separate) balanced amp that I use for headphone listening....been doing that for years.

A solid state amp does not need a load. Tube amps do!

I use an Audio Research REF75 SE with a speaker tao to headphone adapter with my Abyss 12266 TC with no issues. I asked AR technical support before I bought the amp and they said there would be no problem with the amp. They were more concerned with my ears.

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

If you are using the headphone Jack then most integrated amps would not send a signal to the amp portion and there is no problem. Should be easy enough to check. If using the speaker taps then you are finer as others have indicated 

In my 50 years of audio sales, the advice has always been…no problem with solid state, and no good with tubes.  One instance where solid state can have an issue is with amps lacking a common ground.  When I was in college, I had a Quad 33/303, neither of which had a headphone jack, so I bought a quarter inch female in-line stereo connector and soldered it together with some speaker wire and banana plugs to accommodate my Sennheiser 414s. The 303 was designed as a pair of bridged amp modules and didn’t like the grounds tied together…luckily I caught it before it overheated too much!  But if there is a jack, that won’t happen. Another kind of amp that would do that is a class D like many NAD models, but they provide HP outputs at the preamp stage.

As others have stated, running a solid state integrated sans the speakers will cause no damage. Most integrated amps with a headphone port automatically disable the speaker output when a headphone is plugged in. If your HL5's stop out putting when you plug in your headphones, you'll be fine without them. You will also be safe when swapping phones if you have more than one pair.

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?