I am using the 250Ohm version.
The Maestro GMP400 sounds very good, but haven't spent too long with them, mainly using the Grado for tube evaluation.
When a power tube arcs you can damage your headphones.
I suggest you get your tubes tested just in case.
If you are certain what you are seeing is arcing take them out of the amp and stop using them. There is the question of whether the tube is malfunctioning due to a defect or if it is being run at an inappropriate operating point by the circuit it is in, but either way when a tube arcs it is unsafe and can cause damage to the amp, headphones or both.
That is right, a tube tester can not tell if a tube is going to arc over. Most tube testers can not test the tube at the same operating point being used in the amp. You need to take the tube out of service as least for that amp. I had a tube that arc over that did blew out a headphone.
That's right John.
A tube tester may not pick up the problem indeed.
I tested a rectifier with one of the best shorts/leakage testers a Sencore TC28 which didn't reveal any problems.
In circuit it arced after 10mins but I was lucky and my amp didn't sustain any damage.
Yup, I had an amp (Wheatfield Audio) arc a 5998 output tube, and it blew the drivers in my DT990. Good thing they were not on my head at the time, or my hearing would have been permanently damaged as well. It's not to be messed with.
The first time it happened it was a quad of untested TS 5998 from a "reputable" ebay seller.
That was a few years ago.
The second time it was a 5U8C rectifier in my Glenn OTL.
The amp coped very well, I was very lucky.
Sometimes one does have to wonder ;) But I know you know the answer...for many if not most who regularly read this thread, it's the combination of that certain something they provide sonically, along with the mystery/history that collecting them involves...