Don't worry about the source of your woes. The tubes are frequently the source of snap crackle and pop then they just stop unless you haveno fuses in which case a major tube failure will cause all manner of havoc in that case don't wait. Yes input tubes are the slowest to go but they do die on rare occasions, I even bothered to see if there were good subs in case you couldn't find those. There are but they seem obscure.
The output tubes are a matter of preference, but I have heard the KT-88 of choice these days is a GEC repro from Penta labs Chinese yes Penta are considered tops these days. I don't use them in my amps ony because I use EL34L outputs, Also if your amp can take KT-88s then it should tolerate 6550. I understand from my tube preoccupation that the Tung Sol re-issue is a good one. Do yourself a favor and do a search in AA's tube asylum section for both.
The output tubes are a matter of preference, but I have heard the KT-88 of choice these days is a GEC repro from Penta labs Chinese yes Penta are considered tops these days. I don't use them in my amps ony because I use EL34L outputs, Also if your amp can take KT-88s then it should tolerate 6550. I understand from my tube preoccupation that the Tung Sol re-issue is a good one. Do yourself a favor and do a search in AA's tube asylum section for both.