Tube hiss - any idea what's causing it?

Try moving them around. I had some hissing and rotated all - Power and Pre-amp tubes, Left to Right. Checked bias and adjusted as needed. The hissing went away. Thought in my case it might have been an oxidized connection that changing out "fixed". In any case swapping all L to R seems to have fixed things - now for several months.

I switched all the driver tubes around L-R and I still have the hiss. Getting some new EF86s and we'll see what happens. Not sure what my options are if that doesn't work.

The hiss is 99% for sure coming from the tubes. It is very unusual to get the results that Mr. Ghost did and have it go away. That is unless what he describes as hiss is actually something else. Corroded pins usually make more of an intermittent crackling sound than a steady hissing sound.

BTW hiss and microphonic are 2 different things. Hiss is just what it sounds like, a hissing sound that is steady. Microphonic means that external vibrations are picked up by the tube and you hear it through the speaker. If you yell at the tube and you hear your voice from the speaker then it is microphonic; meaning it acts like a microphone. You can also tap on the tube with something like the eraser end of a pencil and if you hear a ringing sound it is to some degree microphonic, but all tubes are if you tap them hard enough and tapping on hot tubes can damage them or cause them too fail so I don't recommend that.

Herman -
I definitely had "hiss"...constant very low level background noise in L channel as I recall. Could really only hear it when the volume was cranked all the way up with nothing playing. It was noticeable however as previously the JoLida had been dead silent. I think in my case, Bless (Erwin) above has hit on it...bad pin connection. Rotating and consequently re-seating all the tubes seems to have made a better connection and fixed things for me.

Jp - Hope you get to the bottom of it and it doesn't prove to be anything major. I'd also check all cables and interconnects. Funny how those things can manage to work loose...but you've probably already done this.

Ok, let's back up here for a moment. The hiss stays the same regardless of where the volume control is set? This isn't unusual since most input tubes come after the volume control. But the hiss is in both chanels, not just in one? Could be that both input or driver tubes have gone noisy at the same time - unusual but not impossible. I would probably contact the manufacturer for some ideas. It could be something simple like the EF-86's or perhaps the 12AU7's but it seems a little weird that tubes on both sides went down simultaneously. If I were you I would probably find some junker tubes (one of each - some end of life tubes would probably be just the ticket) and try them out in the various positions. Could be that the amp is burning up tubes because of power supply issues...