Tweek, the plus is that they have low power. The minus is that they have low power. Low power sounds better because of less gain stages, tubes, and complexity.(All else being equal). Low power cannot drive most of the available speakers to a reasonable level, hence the minus. The art is to assemble a SYSTEM that will work to provide both sound quality and reasonable SPL.
Single-ended is generally preferred for great midrange. Since most SETs have output transformers, they suffer from transformer related problems like rolloff and bloat. The best transformer SETs do a very good job at minimizing this and sound absolutely phenomenal. OTL SETs of which there are very few, can be even better, since there is no output transformer. It is generally conceded that SET is superior in the midrange to push-pull. These are all generalizations and each type can be very good depending on the execution of the design. The sound of SETs like all tube amps is very dependant on the type of tubes used. 2A3 is different than 300B. NOS tubes sound different than Russian or Chinese. Etc. Speaker matching is especially critical. Load must match amp well or you will lose some of the little power you have, and impedance matching is also critical for some of the same reasons. If you get it right, the sound quality will be about as good as it gets. And you will get reasonable listening levels with higher efficency ratings of the speakers. A system like this provides enough detail to be ruthlessly revealing of any flaws in the system or recording. If you don't get it right, you WILL hear it. Many people cannot tolerate this level of detail because of this ruthless revealing nature. It can spoil your enjoyment of lesser recordings. But it will greatly enhance your enjoyment of the great ones. Nothing is perfect. This type of system has its strong and weak points. You will never get "rock show" volume levels. You will get awesome small scale sound. Whatever floats your boat.
Single-ended is generally preferred for great midrange. Since most SETs have output transformers, they suffer from transformer related problems like rolloff and bloat. The best transformer SETs do a very good job at minimizing this and sound absolutely phenomenal. OTL SETs of which there are very few, can be even better, since there is no output transformer. It is generally conceded that SET is superior in the midrange to push-pull. These are all generalizations and each type can be very good depending on the execution of the design. The sound of SETs like all tube amps is very dependant on the type of tubes used. 2A3 is different than 300B. NOS tubes sound different than Russian or Chinese. Etc. Speaker matching is especially critical. Load must match amp well or you will lose some of the little power you have, and impedance matching is also critical for some of the same reasons. If you get it right, the sound quality will be about as good as it gets. And you will get reasonable listening levels with higher efficency ratings of the speakers. A system like this provides enough detail to be ruthlessly revealing of any flaws in the system or recording. If you don't get it right, you WILL hear it. Many people cannot tolerate this level of detail because of this ruthless revealing nature. It can spoil your enjoyment of lesser recordings. But it will greatly enhance your enjoyment of the great ones. Nothing is perfect. This type of system has its strong and weak points. You will never get "rock show" volume levels. You will get awesome small scale sound. Whatever floats your boat.