Yep, that's a difficult frequency - a porous absorber would need to be over 4' thick (1/4 wavelength), even stuffing that into the corners is going to be pretty obtrusive. A sub array would seem to be a good choice if you have the space and wallet.
If you're really intent on treating the room then there are more options. As you're dealing with a narrow frequency band then you may want to look up diaphragmatic absorbers and helmholtz resonators, both of which are more efficient in terms absorption coefficient but have a higher Q (they cover a smaller range of frequencies). Both need tuning to the room so will require a bit of DIY (I've not checked to see if there are any commercial ones available). If you go down this route prepare yourself for quite a bit of trial and error... also grab a copy of the Master Handbook of Acoustics.
I ended up putting in a suspended timber ceiling made up of 1/2" T&G timber, 6" rockwool and a 6" air gap above that... so I lost just over a foot of ceiling height. I had left myself the option of making holes in the timber to create helmholtz resonators but it's good enough without that.
If you're really intent on treating the room then there are more options. As you're dealing with a narrow frequency band then you may want to look up diaphragmatic absorbers and helmholtz resonators, both of which are more efficient in terms absorption coefficient but have a higher Q (they cover a smaller range of frequencies). Both need tuning to the room so will require a bit of DIY (I've not checked to see if there are any commercial ones available). If you go down this route prepare yourself for quite a bit of trial and error... also grab a copy of the Master Handbook of Acoustics.
I ended up putting in a suspended timber ceiling made up of 1/2" T&G timber, 6" rockwool and a 6" air gap above that... so I lost just over a foot of ceiling height. I had left myself the option of making holes in the timber to create helmholtz resonators but it's good enough without that.