Your experience is identical to mine- 4 powerex 9.6V, same charger, 2 years of similarly constant use before they developed problems sometimes causing similar wavefrom symtoms as yours, and sometimes loss of headroom and audible distortion of high level signals. Once I understood the source of the problem and decided to replace them I found the then new Imedion version, was happy with the Imedion AAs & AAAs I'd been using for a number of years and have since been using these for the past 2-1/2 years in the same charger. I now check voltage on them after they have been on for 6+ hours and while still in the powered-up preamps and they measure ~9V. I recharge them and don't worry about using them for 2 weeks. Longer than that and I'll pop them back in the charger the night before. When they eventually begin to drop significantly below 9V after 6+hrs I'll replace them, before I get similar end-of-life problems.