Not sure where to go from here with room treatment

[stephentaylor]

Thanks for your wonderful forum - very inspiring.

I’ve nearly finished a small control room that I’d like to use to mix 5.1 for TV.
The dimensions are L = 5.4m x W 3.65m x H 2.5m, the rear wall leans in by around 15° and there are wooden beams on the ceiling.
I’ve added 16 X 4 cm rockwall absorption panels and 2 chunk inspired corner traps (approx 9m^2 of absorption surface area.
Here here are some photos of the room and some images from REW. I sent a sweep through the center channel speaker (no LFE / SUB) and placed the mic at the listening position.

HR15.jpg

HR13.jpg

HR14.jpg

HR12.jpg

HR11.jpg

HR10.jpeg

HR09.jpeg

HR07.jpeg

HR05.jpeg

HR02.jpeg

I’m not totally sure where to go from here, if I’m interpreting the REW graphics correctly, the RT60 looks smooth and the waterfall seems to show fairly even decay times over the frequency range but the spectrum gets a bit «lumpy » around 225 Hz ( Schroeder frequency ?) and below and I’m wondering how to attenuate / correct it.
I’m also a little concerned by the slight dip around 1k to 3k region but I think this might come from the speakers not the room. (KRK Rokit RP8). I mix at between 75dBC and around 85dBC.
Also I have plans to add more absorption to the rear of the room and I’m playing with the idea of hanging 3 panels over the mix / listening position.
Should I be looking at making helmholz absorption panels tuned to 230 Hz ? and if so how big should they be to absorb around 4 to 6 dB ?

What else could I do to improve the room ?

Thanks for any suggestions.