Tubes making fizzing noise, then no volume.

I've borrowed a beautiful sounding Headphone unit called STAX SRM 007t from a kind friend.

However, last night I turned it on and the top left hand tubes started fizzling/sparking (gently) but indicating a failure.

Now the energiser unit shows all lights and start up procedure as normal, but absolutely no volume may be achieved.

Does this sound like faulty tubes? If so, can one replace the tubes?

If so, can you kindly advise me where to go and sort this out for him and not distress my friend unduly.

Sounds like a bad tube me but after reading the owners manual there is more to it than swapping out the faulty tube. The tube would have to be re-biased. Best to leave it to the Stax service center.
Read the cautions on page 3 and 6
Ask the same question to the Stax headphone users at