I have been a power engineer for a long time. I have personally acquired the following pieces of test equipment:

1. Several scope voltage probes, (one HV voltage probe (V's > 500V))
2. A good oscilloscope current probe.
3. A "cheater plug" which isolates earth ground from the oscilloscope. Helps to preserve ground clips during AC/DC investigations.
4. A network analyzer, if you do not want to do the trial and error method. My book and Ray's are good for estimating the feedback needs, but they do not provide the ultimate answer. Ray's analyzer is great as well as Dean Venable's.

If anyone else wants to make a contribution, please do.