Hifi amplifiers are the heart of any system.
Everything is important, source first of course, but your amplifier must nurture a tiny signal from the sources, switch them silently, multiply the signal without adding or subtracting and then drive loudspeakers of varying impedances evenly across a wide frequency response.
No wonder amplifiers can run to thousands of pounds, not an easy task, and you can hear the difference
Most amplifiers are integrated, everything in one box. At higher price levels you go to PRE and POWER amplifiers, separating the low voltage switching and volume from the brute power amplification part for better sound