Amplifier design is more critical, as are the transformers.
However, the tubes listed in your query are so different that you can discuss the issue because you don't normally find 6550 based SET amps or more modern amplifier topologies using the 300B.
300B: The over the top strength of this tube is its midrange. Treble is severely rolled off. As are the lower frequencies, which are the very definition of loose. Many people feel that the tube's magical midrange is actually a result of a high degree of overexaggeration. In a circuit designed to use a variety of tubes, this is the common feeling. But oh, when things are right, these tubes produce a lovely sound. Very immediate, lifelike, coherent, with a natural warmth and glow that solid state amplifiers cannot dream of reproducing. Moreso than with a lot of tubes, the 300B, even when used in a non SET amp, is quite dependent on the amplifier's transformers and speaker load.
6550: I should preface this by stating I am not a fan of this tube. In my opinion, I don't see much reason to use it, other than availability and some companies like Audio Research saying that it should be used in a retube of many of their amps. I feel that if you want a classic tube sound, use KT88s. If you do not want a classic tube sound use KT90s, which do everything that the 6550 does well even better, coupled with a more listenable and natural midrange. I find the sonics of this tube to be lean and forward; the antithesis of lush. Amps using this tube have often struck me as edgy, unnatural, dry, and lacking warmth. I often felt that owners really favored solid state sound, but revel in owning a tube amp. Most of the time, all of this was corrected by mere substitution with KT88 tubes. The strengths of the tube are its extension at both frequency extremes, stability, reliability, power output, and tube life. Given the choice, I opt for a good solid state amplifier over the 6550.
EL34: Sunny and warm midrange. The sonics in general are warm, lush, and full bodied. This is the tube which most people are thinking about when they talk about classic tube sound. Mostly because it's a tube that was featured in so many classic tube amps(Williamson circuit[which so many tube amps copied or modified], Dynace ST70, Marantz 8/8B/9, etc.). The strengths of this tube are its midrange, liquidity, richness, and non - fatiguing sound which immediately puts most people at ease and can listen to all day long. The bass is loose and not normally so well extended, and the higher frequencies are not as open and crystalline as they probably should be. Those who are into "accuracy" at night probably dream of ridding the earth of the EL34.
6C33C-B: I guess I should list the negatives of this tube before its strengths. The negatives are... Um, well, uh. It's not a widely available tube. Who knows how easy it will be to find 20 years from now, but hasn't that been the story of living with tubes over the past 25 years? It's also pretty expensive for new stock. Further, I believe it's manufacture is in Russia only, as a military tube, it may even be discontinued. It's operating conditions would incinerate other tubes, which would only be a consideration if one needed to switch to a different tube in the future. That's about it. It's strengths seem to be every thing other than what I just listed. It's bulletproof, stable, supposedly lasts a LOOONNNNGGG time. The tube has great power, bass, and extension. Midrange is lush and warm(I guess these would be a negatives to some), and very natural and musical. Highs are open and extended, but airy and smooth. In many listening experiences, I have first said "Wow!", then looked down to see this tube in the amplifier. Natural, musical, relaxed, and effortless have always been the case. Would I be out of line to say that if a tubeophile dreamt up the ultimate tube, this is probably it. Someone could probably rip my description to shreds by saying that amps and companies that use this tube are the creme de la creme.
However, the tubes listed in your query are so different that you can discuss the issue because you don't normally find 6550 based SET amps or more modern amplifier topologies using the 300B.
300B: The over the top strength of this tube is its midrange. Treble is severely rolled off. As are the lower frequencies, which are the very definition of loose. Many people feel that the tube's magical midrange is actually a result of a high degree of overexaggeration. In a circuit designed to use a variety of tubes, this is the common feeling. But oh, when things are right, these tubes produce a lovely sound. Very immediate, lifelike, coherent, with a natural warmth and glow that solid state amplifiers cannot dream of reproducing. Moreso than with a lot of tubes, the 300B, even when used in a non SET amp, is quite dependent on the amplifier's transformers and speaker load.
6550: I should preface this by stating I am not a fan of this tube. In my opinion, I don't see much reason to use it, other than availability and some companies like Audio Research saying that it should be used in a retube of many of their amps. I feel that if you want a classic tube sound, use KT88s. If you do not want a classic tube sound use KT90s, which do everything that the 6550 does well even better, coupled with a more listenable and natural midrange. I find the sonics of this tube to be lean and forward; the antithesis of lush. Amps using this tube have often struck me as edgy, unnatural, dry, and lacking warmth. I often felt that owners really favored solid state sound, but revel in owning a tube amp. Most of the time, all of this was corrected by mere substitution with KT88 tubes. The strengths of the tube are its extension at both frequency extremes, stability, reliability, power output, and tube life. Given the choice, I opt for a good solid state amplifier over the 6550.
EL34: Sunny and warm midrange. The sonics in general are warm, lush, and full bodied. This is the tube which most people are thinking about when they talk about classic tube sound. Mostly because it's a tube that was featured in so many classic tube amps(Williamson circuit[which so many tube amps copied or modified], Dynace ST70, Marantz 8/8B/9, etc.). The strengths of this tube are its midrange, liquidity, richness, and non - fatiguing sound which immediately puts most people at ease and can listen to all day long. The bass is loose and not normally so well extended, and the higher frequencies are not as open and crystalline as they probably should be. Those who are into "accuracy" at night probably dream of ridding the earth of the EL34.
6C33C-B: I guess I should list the negatives of this tube before its strengths. The negatives are... Um, well, uh. It's not a widely available tube. Who knows how easy it will be to find 20 years from now, but hasn't that been the story of living with tubes over the past 25 years? It's also pretty expensive for new stock. Further, I believe it's manufacture is in Russia only, as a military tube, it may even be discontinued. It's operating conditions would incinerate other tubes, which would only be a consideration if one needed to switch to a different tube in the future. That's about it. It's strengths seem to be every thing other than what I just listed. It's bulletproof, stable, supposedly lasts a LOOONNNNGGG time. The tube has great power, bass, and extension. Midrange is lush and warm(I guess these would be a negatives to some), and very natural and musical. Highs are open and extended, but airy and smooth. In many listening experiences, I have first said "Wow!", then looked down to see this tube in the amplifier. Natural, musical, relaxed, and effortless have always been the case. Would I be out of line to say that if a tubeophile dreamt up the ultimate tube, this is probably it. Someone could probably rip my description to shreds by saying that amps and companies that use this tube are the creme de la creme.