https://www.youtube.com/watch?v=Cl9L4XrKvNYOften the DC difference between high and low PS rails is caused by old electrolytic PS capacitors, which have lost capacity or have internal shorts so they no longer smooth the rectified voltage as designed. Or it could be blown output transistor, but you would notice that in the loss of SQ during playback. If you looked inside the chassis you might see clues like burnt components or swollen/leaking caps.
Even a 1VDC offset can cause loud speaker thump, but it may not have visibly burnt parts or swollen caps, yet.
Sometimes the DC offset goes away after the amp is turned on long enough for rails to come up to nominal voltage but if the DC offset is still present on the output even after turn on/short warm up then it can be damaging to your speakers - because the DC will heat up the voice coils in the loudspeaker driver motors, especially on bass woofers (3way) and midwoofers (2way) where there is not a series capacitor to block the DC like there is on tweeters and true midrange drivers.
You can measure a serious longlasting DC offset problem with a DC volt meter on the output terminals, but if the offset is very short, the DMM may not show it before it's gone, then an oscilloscope is necessary to diagnose the severity.
