Being a bit pedantic, but in class A, AB, and B the output devices are not normally described as switching transistors because they are working in the linear region.
Class A, completely linear, and can use two output devices biased on. Gets hot even with no signal.
Class B, two devices push pull. One transistor the positive half and one the negative. Switching between the transistors, but both acting as linear devices. No signal, both devices nearly off so only gets hot when playing loud.
Only class D is actually a switched output, either on or off, hence no watts dissipated in output device and little heat even when running at full power. Very efficient.
Overall, a very good overview of what is necessary for a Hi-Fi Stereo system -
- Cable and Wire as necessary and appropriate.
The next aspect is a system that fits both your Budget, Room Size, and general Requirements.
Stereo music is much more complicated than it once was. Today you have Network Streaming from local storage and from the Internet, CD, BluRay-Audio, though now defunct DVD-A, SACD, and good old Vinyl. A person doesn't need all those things, but one needs to consider which best fit your needs and the content you have. Also, it is possible for a Stereo system to allow Streaming directly from content on your Smart Phone or other Smart Device by Bluetooth.
Then the Room Size, generally not good form to put Monster Sized speakers into a small bedroom. Though that is not absolute, more so just a guideline.
For amp and for most consumers, the working range of amp power is 50w/ch to 100w/channel. If you go much less than 50w/ch, you are getting into economy amps and if you go much more than 100w/ch, you are getting into premiums amps.
So, the deciding factors, before you even start looking at equipment are -
- Room Size
- Personal Requirement for features
- Common Source, that is a source consistent with the content you have or intend to have. CD Player for CD, Turntable for Vinyl, and so on.
All that must be tempered with reasonable expectations. You can't have £5000 desires on a £500 budget. Though pretty much any reasonable budget can be accommodated.