There is a minimum quality requirement for everything, after that it is purely subjective.
Its a shrinking industry that needs to make money just like any other business and if myths and great big fibs can be passed as gospel, deciples will argue until the moo cows come home.
Good CD players - yes, there are
differences between your £39 Eclipse and a £1300 Marantz CD-94. Some engineer spent hours supressing Radio Frequency (RF) interference and getting those zeros and ones to translate to audible sounds with as little jitter
as possible. But not near as much in your cheapie CD/ DVD player.
Amplifiers - a dark art, there are some nasty designs out there, Apache can tell you more if he reads this.
Speakers - basic rules: the more efficient the design, the louder it will be and the less current it will draw from the amplifier. The amplifier does less hard work. The lower the impedance rating (4/6/8 Ohm etc), the more current is drawn form the amp and the harder it has to work, hence higher amplifier outputs are given for driving 4 Ohm speakers.
Theres loads more, some of you will already know, some wont, feel free to ask.