1. Combustion and electronics are two completely separate and non-comparable things. It may seem like a good analogy, but it's not. Real-world differences between cars are easily verifiable under scientific scrutiny. The same is absolutely untrue of claimed differences among amplifiers, even those with $1000+ differences in retail price. I'm not trying to say one size fits all -- you should definitely buy an amplifier capable of driving your choice of speakers at the levels you desire without clipping, and that's all you should really worry about.
2. So the modded unit specs better, but do those specs result in a real-world performance gain, verifiable by double-blind listening tests? I don't often say "charts schmarts," but a drop of .02 (to make up a number) on the THD measurement isn't going to be audible.
3.
"Many people who have considerable amounts of engineering and electrical knowledge cheat themselves out of enjoying the music." And many people who don't fool themselves into spending money on questionably frivolous "improvements". I enjoy my music quite well from my so-called budget system.
If there was no difference between modified equipment and stock equipment, modifications would have ceased a long time ago." Heard of the placebo effect? There are many many snake oil salesmen in the audio world peddling products that even you would agree are absolutely useless. (Example:
wooden knobs) These products exist and people buy them because they don't know any better.
4. Audio memory is short. How can the customer be sure they hear a difference when they can't compare the before/after directly? The human brain is a fascinating organ capable of grand deception. It's wise to verify what it's telling you every once in a while.
