I have owned my Hafler DH-500 for over 36 years. When I got it, it was tested @ 505w into 4 ohms. My only ever problems were once the heat sink got dirty and the amp keep going into thermal overload. Once cleaned, it operated perfect. My other problem was the power switch went bad so I replaced it. Once this amp has warmed up, it sounds today as good as it did back in March 0f 1984 when it was new. I wish I had two of them! Then I'd get two Mono Kits.