Hi guys, been having trouble with my first proper valve amp, a Marshall DSL15c combo.

Initially it overheated or overloaded and managed to have an output valve physically blow up inside, and Marshall replaced this. Before this there was some moderate hissing when I turned from the clean to the distortion channel, but now it only seems to have become worse. Even on very low volume and gain it hisses with this white noise sound, and when I turn up the volume, gain or both it gets very loud. Even found high pitched squealing when I tried to max it. The distortion channel is basically unusable because of this noise it makes!

This issue is only on the distortion (red) channel, clean channel is fine. I'm pretty sure its not due to interfering devices or dodgy leads, and pretty sure my guitar is not the issue either. So do you guys have any ideas as to what the issue may be? Thank you
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