how do i know when my tubes are going bad?

i have a fisher x-202-b with 7591 output tubes. i had the thing gone over last december and have been listening to it regularly for about a year now. lately the past couple times i have listened to it it just didn't have the same sparkle. i don't know how to put it but the past few months have been great. there are times when i listen to my system and just sit there in awe of how good tubes can sound. but anyways the past couple times ive listend to it it sounds good the music is all there but it just is not shimmering if you know what i mean. as far as i know the output tubes where checked but not replaced. i think i needed one 12ax7 when they went over it. also if the are output tubes that are needed any suggestions on what is the best to go with brand wise. thanks.