signs that tubes need to be replaced

Drifting bias is a sure sign however that could also be caused by an out of spec bias resistor.    I had a matched quad of  EL34 power tubes that were about 8 months old.  One had "fire in the bottle" so to speak.  Literally arcs and sparks in one tube... it was scary.  Fortunately Quicksilver amps are robust and it only took out the mains fuse.   That was during play , it was sounding great and then all of a sudden horrible cracking noise through my speakers, I look up and fire....   I was lucky.  That said, I always try to replace before failure.  I have a set of KT88 that have a lot of miles on them and have been my spare a few times.  They still sound ok and hold bias but I'm nervous due to hours on them

I have a new quad of Gold Lion  KT77 on the way.  Hopefully they are as rugged and long lasting as they say

@jchiappinelli 
Look here for additional signs...
https://www.thetubestore.com/how-to-tell-if-your-tube-is-bad

@oddiofyl 
Interested in your update after you fully burn in the Genelex GL KT77s in your QS amps.  I'm recommending them to a friend for his QS mono 60 apmps in place of the stock JJ KT88s.  

My PrimaLuna DiaLogue Premium told me when I had a bad KT88 via the "Bad Tube Indicator" feature.  When a power tube goes bad, the amp goes into protection mode, sound cuts off, and a red LED indicates which tube is the culprit .

I knew about this feature when I bought the amp - and wondered if it really worked.  It does and now I'm a believer.  I don't know if this feature is available on other brands, but I'm glad I've got it on my amp.

No highs no bass and crappy sound this is why i only have tubes(12ax7,6dj8) in my TW phone stage only.Good luck!!

When the silver getter flashes start turning brown at the edges! Eventually all the silver will turn to brown. That is the sign of "old age" - even though the tube will still work!