The Genelex reissues have had reliability issues, which may, or may not, be cured. I had one of my quad short out and there is plenty of other anecdotal reports about these on the Asylum.
The SED winged C seems to be the most reliable of the 6550/KT88 family in current issue, and the KT120, mentioned above by Brf has had great reports, though it pulls a bit of extra current and you should check with cj to make sure that it is appropriate for you amp.
The SED winged C seems to be the most reliable of the 6550/KT88 family in current issue, and the KT120, mentioned above by Brf has had great reports, though it pulls a bit of extra current and you should check with cj to make sure that it is appropriate for you amp.