Power Tube Failure

Obviously a tube can fail at any time so you can never be sure if/when it will. That said, your preamp has a single 6550 power tube which is generally rated to last a couple of thousand hours on average so maybe consider preemptive maintenance and change it each time it gets near that amount of hours. I do that in my integrated amps and tube preamp.

I've found shorts, and weak tubes, once I put a 6sl7 in a 6sn7 socket (or reverse of that) and blew the tube, i.e. dark inside, dead.

I have never had the glass break. It is not how they die.

get thee an inexpensive tester, here's one

https://www.ebay.com/itm/377064722132?mkrid=711-53200-19255-0&campid=5338381866&toolid=10001&customid=2aedea5c-4626-11f1-8ae2-383333336661

always test for shorts first.

everybody's gonna say ..... I'm telling you, you are blind now, get a simple tester at the very least.

When I owned Rogue ST100 a power tube failed and with it took out several parts in that channel. I live only about an hour and a half away from rogue audio and dropped the amp off for repair. I sold it shortly after and never bought another tube amp. 
A properly designed top flight modern solid state component, be it the amp, preamp, phono stage or DAC, is no less musical than a tube component. Especially considering that modern tube gear doesn’t sound much like the classic tubes. SS is more consistent sonically, performs just as well, and imparts way less audiophile nervosa on its owner than tubes. Nothing to roll. Press the button and enjoy music.