Sad to confirm this was , as most would have guessed, a Topping L30 that failed. Topping customer service have confirmed the warranty doesn't include damage to headphones.
To the best of my knowledge it was taken out by a static charge. I am in contact with the seller who have requested I return the unit. I will do this if they cover the return postage but to be honest I will not be looking to use the replacement item I am sure they will provide. I just cannot be comfortable using a device with a known flaw that could take out another set of headphones.
In my view this is a fatal design flaw , especially given the subsequent damage that can result from this amp failing. If I recall correctly, the static charge happened at the end of the day with the headphones unplugged and I was powering down. It was the next day on powering on and inserting the headphone jack that the damage to the headphones occurred. So whatever component failed (and one did, I can smell the damage inside) does not result in the unit not powering up. It then seems to allow DC into the headphones, which is a definite no-no. So not only can the device be damaged by static, my unit remains on in what must be an unsafe state.
I would strongly advise anyone looking to buy an amp to look to something with a longer track record than this. If you own one, I am not sure what advice to give you other than take care and try and discharge any static built up on you before going near the unit.
Edit: by remains on, I mean I can plug it in, lights on etc and happily let it kill another pair of phones. It's not in shut down. I got a cheap pair of phones and tested and they too made the horrific crackles when inserting the jack. I obviously then disconnected it.