Why my system becomes dull after sometime?

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Unless this happens with the exact same piece of music (and I do mean exact), it would be impossible to even say that there is anything wrong at all. If you are streaming and especially if there is wifi used anywhere in the chain, I would suspect network congestion issues to be the problem especially now with networks strained due to so many people working from home and kids at home all using the networks heavily. Also, try removing the power conditioner completely and just use a power strip.