Tube hiss - any idea what's causing it?

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...

Purchased a BAT 300X-SE and out-of-the-box it was hissing like a can of spray paint.
This was a constant hiss (both channels) and not volume dependant.
I didn't pee myself or throw a hissy-fit toward the seller but instead agreed to ship the unit to BAT for repair.
In a week or so, BAT shipped a fully repaired 300X to me and I've lived happily-ever-after. The repair cost me nothing.
BAT said the problem was a "voltage regulator".

I replaced the 12au7s and I still have hiss in both channels. The EF86s are coming next week. If that doesn't clear things up then it's back to the manufacturer for a check up. Perhaps it is the voltage regulator.

Many thanks to all for chiming in on this issue!