I started on Audioholics with an old 80's cheap, 2 channel 150W amp and old speakers. Upgraded to better old speakers and the improvement was as good as anything that I have implemented to where I am at now. Upgraded to newer speakers (still with the old amp) yet that follow a different style than the new/old speakers and it changed things yet again, for the better in different ways as far as options.
Upgraded to a much newer AVR (albeit antiquated Denon 3805) and the most marked improvement was adding subwoofers and bass management. In spite of the newer amp section, the most profound, audible changes has been speakers, and subwoofers. So much so, that I have been on a path to build different sizes and types of speakers and subs, almost completely forgetting about what amps I am using. The reason being, I enjoy listening to different types of speakers. I have big fat ones, tall ones, bookshelf, desktop etc. and these all change how I interact with my room for a specific quality that is mostly based between low level and loud listening. Still, the amp has a rating (IIRC, see, I have forgotten) between 120-160 WPC depending on load and I have not been able to use all of that power with the speakers I own in this 12' X 20' room.