It should not be a problem just make sure it isn't biased to hot. The 6550 and KT-88 are almost interchangeable the sonic are quite different.
I personally found glory in the reissue Gold Lion KT-88s. Use a good dealer like Jim McShane a sponsor on AA tubes forum, he is careful about matching and screening the tubes for losers. They cost a pretty penny for a non NOS tube, don't be shocked, but they spank everything else in the amp I am using. (Jadis DA-60 )
I personally found glory in the reissue Gold Lion KT-88s. Use a good dealer like Jim McShane a sponsor on AA tubes forum, he is careful about matching and screening the tubes for losers. They cost a pretty penny for a non NOS tube, don't be shocked, but they spank everything else in the amp I am using. (Jadis DA-60 )