Do you have a compressor in your chain? I can't comment about the Bassman specifically, but that definitely sounds like excess gain from the boost causing some kind of compression to engage.
If that does prove to be the case you could try a boost pedal with separate gain and volume controls, to see if there is some combination of those settings that works better for you.
Could also be an interaction with some other pedal or element of your chain, but it's hard to say without knowing what all you have going on.
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