Speakers sound REALLY different. IMO, if one compares different mid-end AVR's and source units the differences are small compared to speakers. Speakers can sound SO different that you really don't have to think about it or concentrate on it sometimes...its is just blatantly obvious the difference between a set of Polk vs. Klipsch vs. KEF, etc.
Then once you jump into the world of DIY and put together something nice this experience just opens up the "never satisfied" door. Some just have to the need to tweek!