FWIW, regarding Reprince's comments and Tvads concurrence, in my experience the 6550s (SED's especially)is similar except I would describe it a little differently. I consider the 6550 to have a more linear sound with a softer high end than the KT88 which I have found to have, in comparison, a bit of bloom in the upper bass and some 'sparkle' in the upper mids and highs. The KT90 and the 6550 are somewhat similar except the KT90 is not short of high end energy. Which of these tubes sound best greatly depends on the amps they are used in, and, obviously, your system and tastes. I use both of these tube types in two different amps and neither tube was optimum when used in the other amp. Called system synergy!
One caveat for those considering 6550's who are seeking 'warmth'. You won't find it in 6550's (or KT90's). EL34's and 6l6GC's (if you can use them) are a good alternative for mid-range, and the 6l6GC has a softer upper-mid range highs. Quite a nice tube actually, but neither of thise have outstnding bass.
Mt T if you can do without the bass of a 6550 the 6L6 has tone you should love. The Mc probably will not use it though.
One caveat for those considering 6550's who are seeking 'warmth'. You won't find it in 6550's (or KT90's). EL34's and 6l6GC's (if you can use them) are a good alternative for mid-range, and the 6l6GC has a softer upper-mid range highs. Quite a nice tube actually, but neither of thise have outstnding bass.
Mt T if you can do without the bass of a 6550 the 6L6 has tone you should love. The Mc probably will not use it though.