new to tubes

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.

I would not be able to make that description. However, seeing as how you just bought the amp, why not listen to it as is for a few months?

EL34s are closest to solid state sound of the three tubes...clear and articulate, not as much bloom or romance as the KT88, truer timbre and more accurate tonal balance, a tad more clinical, but without the transistor signature or chip sterility of solid state. I do not agree that the EL34 has diminished bass vs. a KT88. The bass of the EL34 is tighter and more accurate.

Of these two tubes, the KT88 offers the largest soundstage and is perhaps the most colored tube...technicolor...compared to the EL34. The KT88 is more euphonic, more tube like, and a little less accurate.

By the way, I own an Audion Silver Knight PX25 amplifier. I have come to really appreciate the PX25 tube...