If you inspect a Fender valve amp that is north of 15years old, look at the electrolytics carefully. If there are signs of leakage on or near the positive lead , with double ended types, it is time to replace them. Snap fit and single ended capacitors always show signs at the business end. Some say they are swelling when the plastic cap on the base (the non business end) swells but that is due to the plastic ageing, nothing more sinister. Visually inspecting coupling capacitors is more difficult. They all look good even when they leak, electrically. The old school way to check capacitors is to use an AVO model 8 on high Ohms range and check for leakage. This sometimes doesn't work though as the meter only uses a 15volt battery on high Ohms. A DVM is no good at all. A valve tester is nice to have but very expensive unless you can warrant the expense of course and then an accurate one is hard to find. My AVO 160 will read the beta values as well as current flow for a desired bias voltage etc and is very accurate for balancing sets of output valves. The Blues Junior, for instance had a bad batch of electrolytics and they physically leak after a few years. Then again one gets to know these things with the experience of time. To find an incorrect value component is one of the most difficult things with regards to stock amplifiers, you have very observant eyes. Best regards and keep up the good work.