A comment: Everyone wants to find something to blame. They want every failure of an amp to be BECAUSE of something. They also like to cite past performance. This worked for 20 years, why would it stop now? Well, life isn't like that.
Everything works until it doesn't, simple as that.
But it happened when I as PLAYING??? Of course. Most failures occur when something is ON and in use.
You had a tube failure. Tubes, new or old, can fail at any time. Like a big bug can fly into your windscreen at any time. They don't only do it on Tuesdays or only when the Everly Brothers come on the radio.
You may have an FX pedal that makes 20Hz, but that doesn't mean your amp can produce it. Never assume. I'd be willing to wager that your tube would have failed even without the FX. It simply ran out of time on this earth.