Would You Rather Own A Good SET Amp, Or A Great Push Pull Amp?

How can an audio system be accurate if it isn't loud like some live unamplified music is?

@invalid It wouldn't be! It must reflect that of the signal (since that is as close to the music as an audio system can get). 

If the system sounds loud when it really isn't (for example, only 83dB), that should be a point of concern. 

are there other important parameters based on which we can judge about the SQ without the audition? 

@niodari  Yes- I mentioned some prior, such as understanding the harmonic spectrum created by the amp. IMO that is more important than the THD. 

The loudness can be subjective.

Exactly my point; it can be independent of the actual amplifier power due to the presence of distortion fooling our ears into the impression its louder than it really is (as seen on a sound pressure level meter). 

All analog tape machines have distortion but I think you drew an incorrect conclusion from your cassette anecdote. 

If you really want to hear what a 5.5 Watt SET does, you would need a speaker of at least 12dB more efficiency (in most rooms); the few dB difference you mentioned still means that the amp is being pushed too hard.

I know of a PP amp that has about the same power as your SET. I've seen it take on SETs of similar power with no problem. It, or something like it might be something to try.   

Not that I am an educated individual in such matters, but I do know how to learn basics. 

A Stray away from the tech' talk and back to basics what I know. 

I know through being involved in and being close to design talk and builds, that certain circuits are produced with the Trannxs selected to be used to enable a Valve to work at a Optimised state.

Creating the different circuits to suit the Amp' design has great potential to create discernible difference to an End Sound. Neither should be any less enjoyable as an experienced End Sound, which is what I am experiencing. 

Yes- I mentioned some prior, such as understanding the harmonic spectrum created by the amp. IMO that is more important than the THD. 

@atmasphere ,  it would be interesting to detail this point: which are these important parameters that more realistically reflect the real performance of an amplifier, or are they still to be developed? 

I am about to change my earlier  point of view that engineering is a purely applied science. I share the concern by @invalid . I think that before we talk about the amplification, we need to understand how the music performance in question was recorded. That is to say, two things are to be distinguished and seen  separately: Which kind of music we are talking about and how it was recorded (with a single microphone or each instrument separately etc.) , and then how the recorded music is reproduced by some amplifier. I think that the second issue should be discussed based on the analysis of the first one. So suppose for now that we have a "perfect" amplifier of your choice and see how its reproduction is affected by a particular kind of music and by how this music was recorded. 

I suggest that an inherently loud music (e.g., rock) is easier and more natural to reproduce loudly than an inherently non-loud music (e.g., jazz). If we force the amp to play an inherently non-loud music loudly, you may unavoidably get some distortion, regardless of how the amp is built.  By the way, among the two cassettes mentioned in my  previous post, the first one was jazz and the second one was rock music (such an analysis would understandably make less sense for CDs, i.e., it should be an analog source). 

I suggest that an inherently loud music (e.g., rock) is easier and more natural to reproduce loudly than an inherently non-loud music (e.g., jazz).

@niodari All electronics don't care what kind of music you put through them, which is to say there is no way an amplifier can favor a certain genre of music.  The level of difficulty is thus the same for the amp regardless of the music.