Seems like amp overheating, decreasing power to prevent frying itself. Dunno - add cooling fan to it?
The most reasonable solution would be to live with it. If you like music you probably have to spend some money to get what you want.
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2023 Kia Stinger GT AWD - as a family car...
2022 ZL1, M6 as daddy's ass mover
Charger sold! Better to sell, than waiting to be stolen...