Timbre Tuning the Sundara

[Jimbob54]

My advice is to simply use tape and cardboard. Experiment with covering the outside grill starting with the area you've identified as subjectively pleasing maybe stick some cardboard to them. You'll look like a idiot but it will be a fun bit of hobbying for a afternoon.
Eq will not be able to recreate the reflections caused by your hands distance and angle.
A similar effect happens when listening to speakers. Cup hands behind ears and spl in higher frequencies increases loads as does stereo effects. Again looking a idiot is the side effect

Buy an XL beanie hat to go over the cans and the cardboard.

Or mess with crossfeed / spatializer type DSP to see if you can recreate something akin to the hand cup effect.