How do I troubleshoot distorted or weak sound from the DSL40CR?

Start by adjusting your Gain and Volume controls to ensure they are set appropriately for your desired tone. If the issue continues, the valves may be worn-out and require replacement by qualified personnel, or the speaker may be damaged. In either case, contact qualified service personnel for assistance.

From the official Marshall DSL40CR Tube Amplifier manual, page 7

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