there is few classes of amps.. Class A amps and Class D amps circuit design are more popular.

basically those cheap amps are not actually classified in these catagories. they are using amplifier chips only... i've dismantled a so called 400W PMPO chicken brand amp that costs RM100. the chipset they use cannot even pump more than 5W RMS

my friend got a so called made in USA amp that he uses to blast 2x10" also made in usa sub woofer. the rated power was about 1200W PMPO... it work flawlessly. but once i put an 8" sumo polaris dual coil woofer replacing the 2x10" woofer, the amp cut off in less than 1 hour play. FYI the sumo polaris is only 60W RMS per coil. so dual coil is only 120W RMS with 700WPMPO.

got him a second hand audiostream amp for about RM600 and it played flawlessly.

expensive amps have proper circuit design and better quality components. Class A amps is normally used for woofers, while class D amps which are much more expensive and harder to produce is for the sound quality.