Bass Traps

My understanding is that they are somewhat narrow, but will absorb some surrounding frequencies also.

That's the limp mass plan I've been looking at. Thanks, I'll have to do the math and find my nodes. Room sweep analysis is just showing slow bass decay, not any consistent peaks.

I bought this stuff for the limp mass.  It’s better than the vynil stuff:

https://www.ebay.com/itm/Neoprene-Rubber-Sheet-Strip-1-4-Thick-x-24-wide-x-10-Feet-long-FREE-SHIP/281773665932?hash=item419b04ee8c:g:aOQAAOSwuLZY4J0t

i think i used 1/8” for 63hz, and will use 1/4” for 50hz.  It all depends on desired cabinet depth.  Thinner neoprene requires deeper cabinet.

I just spend an ABSURD amount of time researching this. I landed on ASC IsoThermal TubeTraps. They have built-in diffusion (or absorption)!

• Extended bass absorption below 250 Hz
• Treble diffusion (or absorption) above 250 Hz

https://www.acousticsciences.com/products/isothermal-tubetrap

Whatever I do, I don't want to mess with anything above 250Hz. If I do, my room will sound dead. 

Just gotta tame that bass!

OP, sorry for the hijack. Thank you @auxinput and @hifiron for the recommendations and advice