Acoustic treatment

I need some advice on how to proceed with acoustic treatment in my listening room.  Setup is Wharfedale Jade 3, Cambridge CXA-81, Bluesound Node 2i, SVS sub.  Stands are damped with pea gravel and sit on needles on metal disks.  Speakers sit on dense foam isolation pads.  They are 30" from the front wall and 40" from the side wall.  Room is about 15x13x8.  On certain music (Hawaiian slack guitar, some soft piano jazz) I seem to get resonance around the bass/mid crossover at 350 hz, perhaps a little higher.  Otherwise I am very pleased with the sound.  I have acoustic panels on the front wall behind the speakers and two DIY tube traps that I've moved to various locations.  Here is my question:  assuming that the room is the problem, should I look in the direction of absorbtion (i.e., a resonator tuned to the 300-500hz range) or dispersion?  Thanks.