new to tubes

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...

Listen to the PX25 tube...there is no doubt that this tube has more grunt...it is faster handling transients than a 300b, and not as warm, nor is the bass response as rolled off. If you listen to tubes within a line, you will find that every line imparts some bias...Cary and Audio Note are darker overall than Audion. But there are characteristics of each tube that transcend the lines, and may be magnified in some product lines, and diminished n others, but those characteristics define the sound of the tube. You need to decide what is important to your ear. If you like hard rock music, perhaps the EL34 or PX 25 would be a better choice than a 300B? Vocals and string instrumentals, acoustic guitar, the female voice, perhaps the 300B is the tube for you. Big soundstage, technicolor...maybe KT88s. Just make sure you listen to the tubes in the equipment that you intend to purchase, including the speakers and the front end. And you owe it to yourself to listen to Single Ended amplification before you decide.

Sorry, but you cannot use teh PX25 or 300Bs in your system. I was just using the 300B and PX25 as an example of tubes that have generally acknowleged sonic signatures, because the previous comments stated that the sonic signatures were totally equipment dependent, and that is not true. With the tubes that you have mentioned, the EL34s will give you the best mids, the KT88s the biggest soundstage and a technicolor presentation, and I am not familiar enough with the 6550s to give you a fair assessment.