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Amp placement in a room has an influence on bass due to boundary interference, this can both boost bass and reduce in different places in the room due to phase at various frequencies. Also raising the amp off the floor and placing it away from the rooms corners can help reduce bass.
This is something you may also want to look at while sorting out the bias issue - the bias voltage supply circuit capacitors as posted by TimB https://www.marshallforum.com/threads/so-my-dsl-100-jcm-2000-died-today.125845/post-2247999
@bobbyvasco Listening to your video - that is not a hiss - that it is feedback. It normally occurs when you are to close to the amps speakers / high volume in a small room. And it can happen if the Master volume pot is oxidized at the position that it's set at, resulting that the pot wiper not...
@ledveder - It sounds that not all of the input signal is reaching the grid of V1B or even bypassing the tube. Also check the solder at the grounded end of R34.
From the schematic it seems that the size of the mains transformer allows 2 ohm speaker load to be used safely and create some distortion in the output stage.