If the amp has KT88s in it, you can directly substitute the 6550s but be careful about using the EL34s. You didn't say what model amp you have but the EL34 tube usually requires a little circuit modification if you want to use it. Follow the manufacturer's instructions very closely on this point.
The KT88 tube has a nice liquid euphonious sound. The 6550 tube may have a bit more solid bass and be a bit less "3D" in my experience, but it is also supposed to be a more rugged tube.
The EL34 is known for a wonderful midrange but tends to be lower power and without the bass and treble extension of it's bigger brothers.
Of the tube amps I've had in the past I think the KT88s were my overall favorites, but we're talking pretty subtle differences.
The KT88 tube has a nice liquid euphonious sound. The 6550 tube may have a bit more solid bass and be a bit less "3D" in my experience, but it is also supposed to be a more rugged tube.
The EL34 is known for a wonderful midrange but tends to be lower power and without the bass and treble extension of it's bigger brothers.
Of the tube amps I've had in the past I think the KT88s were my overall favorites, but we're talking pretty subtle differences.