the most expensive and subject to the most cost cutting will be the power amp section. Typically the power supply is a very large transformer (high cost), several very large energy storing capacitors. heat sinks, power transistors.
Receivers/avrs are very cost effective in terms of functionality provided, but cost cutting will occur in the power amp section. The cost cutting would manifest itself as the amp not delivering the current required to meet output specks. Most loadspeakers hit a low impedance somewhere near 100 hz, Thus this would result in your 100 wpc delivering say 20 watts. It would be noriceable,
Take the preamp out to a decent 200 or more wpc amp and you may here a substantial difference, dependent upom the speakers.
Thus when people say that speakers are the most important to upgrade, there is an asterisk that should say “dependent upon power amp specs.