This is what I have learnt about hifi.

1. The room is the ultimate limiting factor - It governs the overall sound of the system.
2. The speakers need to be able to drive the room - If they're not up to the job, you might as well be pissing in the wind.
3. The amp needs to be capable of driving the speakers - See 2.
4. All transports sound the same -IMO.
5. DAC's do not sound the same - As CD/DVD player analogue output stages (or any other analogue output) don't always sound the same.
6. Cables don't make a difference - Unless they measure significantly differently.
7. DIY hifi is ultimately more satisfying - You just get more for your money.
8. There are many snake oil salesmen in the field of Hifi - Their ultimate goal is to part you from your hard earned by convincing you your hifi will sound better, when infact, chances are it won't.
9. Expensive Hifi may sound different - But not necessarily better.
10. By far the most important thing is that the music itself is the most important part of the Hifi system - Great music, even on a shit system, can stir the emotions. A fantastic system with shit music is just a fantastic system, nothing more.

All of this is just my opinion, which I have formed through experience and experimentation. I collect music, not hifi. I have put together a system which allows me to enjoy my collection of music. Nothing more, nothing less.