Overlooking factors like impedance matching between preamp and amp or between amp and speakers..... Or cartridge and phono stage......
Or using underpowered electronics in a large room. (This often occurs with demos in which the person demo'ing the system normally plays the system in a smaller room.) Or misdiagnosing a sonic flaw in which making a change in attempt to mitigate the flaw ends up making things worse........
Maybe the biggest one using a source that isn't all that great (or if a turntable, not set up optimally) or not paying enough attention to the pressings being played...... (I've encountered demos on YouTube of megabuck systems playing noticeably compromised audio tracks..... )
There are a lot of oversights that can occur in system building, in which putting more money into the system alone will not necessarily cure the resultant sonic disappointments.
