Bob, how did you determine the one corner bass problem? Boom...? Excuse my naivete. BTW, I have a similarly "cosmetic" treatment in my room and a rug covering the 1st reflection on the ceiling (yr cellulose is far better, I'm sure).
Rdr: why not try the usual decorative stuff on side walls, doors & windows: art, small carpets, heavy curtains for doors -- that look nice and absorb... perhaps, some of the skyline panels could be used for side or back walls with a suitable "cosmetic" touch (fabric cover?). I remember Abstract & others have been quite inventive in this respect with household items.
Good luck!