I thought this thread was dead too, but I guess not
Anyways, I'm not sure if anyone mentioned this but it also depends on your listening equipment. A high-end receiver/amp/whatever probably won't sound much different than a low-end amp if you're using low-end speakers/headphones. You have to scale appropriately for whatever you have. If you're powering a set of Onkyo HTIB speakers with a high-end amp, I'd say it's safe to assume you won't notice a world of difference as if you were powering them with the receiver they came with.
Also about the engineering going into electronics and whatnot; there's a hell of a lot of it. Something as simple as the gain on an op-amp can change the entire output of the system. These electronics aren't just slapped together all willy nilly; there's a LOT of math behind it all. Even something as simple as adding a capacitor to eliminate a little bit of noise can go a long way... especially if that signal is being amplified to 10x or 100x it's original value. There's also the reason why people have careers as engineers in the audio electronics field, spending their whole lives learning about it. You can say the same for anything. Similar to my car analogy... would you expect something made by Toyota to perform better than a car designed by some third-world knock-off company without the proper engineering?
You're paying for the talent that went into that piece of electronics. Onkyo can tell their employees to design an amplifier but they want it to go from concept to product in less than one month. There are compromises that have to be made in order to get it out the door. Not to mention the fact that components are more often than not selected with price (rather than performance) in mind. A company will select a resistor with a 10% tolerance over one with a 1% tolerance just to save a few cents per unit made... that doesn't seem like a lot but think about it when they're producing thousands of units per week. Now a high-end company can tell their employees to design an amplifier but they can spend four months on it. This enables them to put more time into the design, review, testing, prototyping, etc. etc. which will produce a better product. This same company may also restrict all their components to specific tolerances, without pricing in mind, because they CAN charge a lot for each unit made, thus making up the difference in component cost.
It's like the CPU example. An AMD CPU might have a front-side-bus of 800MHz but the Intel could be 1333MHz. The average consumer won't ever need to care about this but in a high-end demanding application, this will make a difference. Same with hard drives. Sure, there are 15k RPM SCSI hard drives that are wicked expensive and store maybe 320GB, and there are 1.5TB 7200RPM hard drives that are much much cheaper. For the average consumer and most of the people out there (myself included), the 1.5TB is the logical option. But for a company/individual who runs a server, the 320GB is what they're going for. It's the same for audio. The average consumer just wants something conveinent, easy to use, and sounds decent. Now people on this forum only want the best, and that leads to more expensive products. If you loved movies, would you buy a cheap Olevia 40" LCD or a more expensive Pioneer 52" plasma, or would you go all-out and dedicate a room to a 150" projector?