You don't say where you have tried the 3 3 up front and what xo you have set on the 3 3 and AVR, also are you using MO or ME on the 3 3.
I would try front left corner where the ceiling is at it's lowest point, 3 3 xo out and AVR xo set to 80hz.
I also have an acoustically challenged room, no external eq or room treatments. It took 2 mbms upfront with mains and 3 3 behind couch to achieve best freq response and sound.
I just use the HSU CD, RS SPL and Excel to take measurements.