my tweakend

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antoine thats my lovely rooms modes!

nothing to do with the subs gain unfortunately

the new room will have enough treatment to combat this

Are you sure it is a room mode? As I mentioned it looks to me as though it's a result of placing the microphone too close to the back wall. At that position you will see an excess of bass due to constructive reinforcement across a band of frequencies (that is, not a room mode as such). Having said that, if that's your listening position you can maybe treat it in a similar way to a room mode anyway.