Below, I address the list of topics from another thread, "amplifier truths and myths":
1. Transformers: The bigger “iron”, the better, hence massive power supplies and output transfers sound best.
True that more metal saturates less easily, but the shape of the core is also important. This is why "pancake" toroids are harder to design. Also, once the transformer is "big enough", making it bigger doesn't add any value other than more weight. When transformers are loaded heavily, they put out less voltage, and the power supply rails fall. If the amp is designed to take this well, you can minimize the size without any noticeable effect on the sound. Toroids are preferred to E cores due to stray field issues. For output transformers, the thickness of the windings is very important. The bottom line is that there are so many factors at play, you have to trust that the designer took everything into account. I mentioned previously that OTL amps are preferred, but with a good output transformer design (yes, beefy), you can get good sound from tubes.
2. Rectification: Tube rectification is better than diode, preferably with a choke in PS
Regarding the choke in the power supply, this depends on the design of the choke (core type, inductance, etc.). Too much inductance can cause the rails to "bounce". Also, It's too general an issue to say what's best here because every implementation can be done right or wrong. I can't think of a solid reason to say tube rectifiers are any better than silicon.
3. Tubes: NOS is generally better than today's Chinese, Russian, Czech and former Yugo production. It's more than just testing well.
No comment. I'd rather leave that question to a good tube amp designer, like Roger M!
4. Parts I - Caps : Good ol’ Sprague caps sound better than new expensive exotics
New electrolytic capacitors are very reliable, have low resistance, and are priced much more reasonably. Exotic caps in power supplies might be a waste, and it can be a better alternative to use standard ones with good bypassing instead (ceramic, poly). There's no such thing as "magic".
5. Parts II - Materials: Silver is better if you can afford it. Silver transformers, wiring, etc…
Better specifications and reliability are the key. There's no cheating physics.
6. Design: Fewer stages the better
Too broad of a statement, but in general, this is true. Imagine that every stage of an amplifier contributes its own distortion. The end result is a product of the combined effects of all stages in the signal path. However, I'd rather use three stages that have 0.01% THD than one that has 0.1%.
7. Negative Feedback: Preferably none. Zero NFB is best
Well, once again this is about the end product and too general of a statement to be unconditionally true. Feedback does create problems, but if done properly, it can solve more problems than it creates. If you can make an open loop amp with no distortion, noise, or output impedance than you don't need feedback. However, this is not technically possible. I might venture to say that feedback design (a very involved topic) is most important to the sound of an amplifier. Also, as output power increases, the need for error correction generally increases with it.
8. Design II: Class A … nuff said, generally sounds better than AB, B and other variants
Class-B is not practical for audio due to crossover (neg to pos) distortion. Class-AB is not as good as Class-A in this same respect. The biasing of Class-AB can put it close to Class-A, but than you throw more power into the room through heat than into the speaker, especially at low volumes. Class-H and Class-G are higher efficiency, but suffer the same problems as Class-AB. I personally like the sound of Class-A, but it can be as much a space heater as an amplifier! My favorite (big surprise) is Class-D. Beware of fake class names like Class-Z and Class-T because they are really Class-D. Class-D amps are often called "digital amplifiers", but this is a gray area as far as correct nomenclature.
9. Construction: Hand wired is generally better than PCB
This really depends on the quality of the wiring. Complicated designs are not "hand wirable", so I think this is a false statement. PCB layout is an art in and of itself, and if done properly is better electrically. This wasn't the case in the 1940s.
10. Straight wire with gain : Tone controls are generally bad, another complexity to degrade the signal from "purity"
Same as with more stages in the amp. The less the audio goes through on the was from the source to the speaker, the better, but it's the compound effect that matters in the end.
11. Transformer II: Transformer-coupled amps perform better than those using a cap between stages
I believe direct coupling is best (servo inputs, etc.). Frequency response at the low end of the spectrum can be ruined by caps or transformers. However, if designed properly the effect of good quality passive components in the signal path is nearly zero.
12. Made in the USA: The Chinese don't care about quality of products. Made in the USA is best if you want your amp to last.
In general, yes, but I can't say if this will still be the case 20 years from now. The Chinese are getting better at producing quality electronics every day.
Please understand that I had to limit how much time I put into this post, so I couldn't address every nuance. The bottom line is that you can't make conclusions based on broad topics, so just listening is still the only way to know who makes a better amp. I believe that designer experience is the most important thing, not brand name, magazine reviews, or gimmicks! Thanks for reading this, and I invite comments!