The idea for this thread came from Tasha and I think it's a good one. All rules are numbered 1:

1) Always try in your system before you commit to purchasing.

1) Never try anything at home that you cannot own.

1) Trust your ears. Measurements can be a useful guide but they only tell a very tiny part of the story. Manufacturers' own measurements are not always to be trusted anyway.

1) Don't shoot the messenger. Respect hierarchy and synergy. If an amp or cable sounds worse than what you are used to it may simply be telling the truth regarding what is further upstream. If you hear little improvement there may be a bottleneck somewhere in your system. Plan your upgrade path accordingly.

1) Setup is important. Modest yet well setup systems nearly always outperform exotic systems that are just thrown together.

This list is not exhaustive. Additions and comments are welcome.