Where did you place the tub? It needs to be in an anti-node of the mode to be effective. Do a quick check here:
hunecke.de | Room Eigenmodes Calculator
And don't expect miracles from helmholtz, their effective absorption area is around 2.4x mouth area.
Edit: We have an array of helmholtz resonators in the corners of our tracking room, they work very well there. But it's an entire array, floor to ceiling in two corners.
Edit again: Sorry, I just read your placement.