Or, it's just a typical Behringer product, and has reached the end of it's short lifecycle...

Seriously, it MAY be a tube, but I can't see a preamp tube going bad in one of those, since there's hardly anything going through it. Preamp tubes last a good while in a hard-ridden tube guitar amp, and those hardly get ridden at all.

And don't be fooled into thinking it's working, "because it glows". They don't glow, and never did. That "tube glow" is LED's behind the tubes.

I'd guess it was some other component, or bad workmanship like loose solder joint, etc.

Kapt. Krunch

Behringer tube ultra in pro recording? Try posting this in the Budget Gear forum, not trying to be a dick, but you'll get more help asking the right question to the right person. You'll find more people in that forum that have experience with the 1953t ultra gain.