For the anal retentive audiophile...a mains frequency tester

You might as well include some test equipment from Fluke Instruments so as to detect the presence of harmonic distortion on the AC line. The 5th harmonic in particular can be quite pesky causing power transformers to vibrate and run hotter and overall power supplies to not operate quite as designed. If you have this then a good power conditioner can be quite helpful- if its able to eliminate the 5th harmonic. Most so-called 'high end audio' conditioners cannot (the PSAudio can and is one of the very few).