Those terms mean nothing though - what does "resolution" even mean? Can you measure it? Can you quantify it? Can you reproduce it and create a box with a dial on it that you can screw with to turn "resolution" up and down? The same for the other terms.
That's what I'm asking myself! If a ss amp is simply amplifying and not adding or subtracting anything to the signal, then a $100 will definitely do as well as a $1000 amp. So what's the difference other than the price tag? Is there an extra %1 or 2%....or more.... of something that you get for the extra money? More clarity....resolution....better sound stage, separation, imaging, all those audio terms that people like to throw around.....is there more, or better, with the $1000 amp vs the $100 one?
I mostly agree with this. Where I'd disagree is basically that, if you cut the super insensitive (makes them sound like jerks, doesn't it?) cans out of the equation, you can drop another ~$200-$300 off the top and be through with it. But yes, the EF-5 should really do everything for all things. So should most competent receivers. The "hiss thing" isn't always a matter of quality on the amplifier's part - it's partly (mostly?) on the headphones as well - for example my in-ears will hiss with more or less anything you plug them into (including some uber-expensive boxen), but if you throw an impedance adapter in there they shut up (or if you switch to another headphone). All amplifiers actually have that "hiss" - http://sound.westhost.com/noise.htm
Some do slightly better than others, but ultimately once you get to the "good enough" bar, it stops being a problem (there is no magical "slightly audible" realm for the golden ears). e.g. - I can report that both my E9 and VHP1 will hiss into my IEMs, but both are silent with my SA5000s.
Finally, low output power doesn't mean noise - there are plenty of very clean and very low power devices out there, like the Sony Walkman mp3 devices, or the iPod, and so on.
Lower powered amps, which are usually cheaper, are more likely to introduce more noise, especially when you really crank the gain. All of my low end budget amps have higher noise floors than the bigger ones. As I mentioned before, my Little Dot I+ will drive my HE-6s on high gain with the volume all the way up, but there is a noticeable low level hiss when there are silences in the music. That might annoy some people, or it might not, depends on the person. That noise isn't there with my monster Kenwood or my EF-5, but the LD wasn't much over $100, so I don't expect it to perfectly drive the most difficult headphones in the world. Would an O2 do the job with them? No idea, I don't own one. However, I can say with a high degree of certainty that if someone were to look at their needs in a completely unbiased way, they'd likely see that an amp like the EF-5 ($400) is all they really need, no matter what cans they buy.
Edit: Oh, and sometimes people just want to buy a nice amp because it looks awesome. That's a completely acceptable reason, IMO. It's the main reason I want a big honkin' tube amp, they look bad ass.