I was advised to look between the pins and if there were dark spots between them then it was time to change out. I don't know how valid this info is, but I did have some Bugle Boys that sounded soft. I pulled them out turned them over and lo and behold dark spots between the pins. Go figure. Best of luck on your search.
How can you tell when tubes are shot?
I have an ARC REF1 and ARC REF Phono Preamp as well as assorted solid state gear. I leave everything on 24/7 and am wondering if the tubes just quit working (ie: get really noisy) or whether there is a gradual decline in their performance? ARC makes recommendations on who long they should last but I have been running the REF1 steady for over 18 months and haven't noticed anything.