For me, the hardest part has been budget, and I've resorted to older used gear. With two SVS PB10's, one Mirage S12, and the graphic EQ in a TX-NR929, I've managed to get subwoofer response down to 15Hz within +/- 5dB of target slope -1dB/octave.
The second hardest part is the room. The system is in a 2000 sqft open floor plan with 8' ceilings, a stairwell up front in the living room, and two passages off the side into the kitchen. The room acoustics filter everything below 30Hz at the MLP for all channels in the front of the room. I think part of the filter is from the open boundaries up front and part is from the enclosed rear area behaving like a tuned trap, but I have no easy way of verifying these suppositions.
I extended the subwoofer response by:
1) placing the subwoofers in the relatively enclosed rear dining area where there's some boundary reinforcement. This avoids the filter below 30Hz.
2) tuning the placement so that all three subwoofers have flattish and smoothly varying phase at the MLP. The Mirage is in the center of the rear wall and the SVS are at more or less the 1/4 point forward of the rear wall along the side walls. I had to avoid the rear corners because the left rear corner has both phase and amplitude issues, and for localization reasons I prefer left-right symmetry in subwoofer placement. I don't notice localization with front-rear asymmetry as long as I am at the MLP and facing forward.
3) aligning the phase settings of the PB10's so that the middle of the sub bass (50Hz) lines up across all three subs. There's a slight modal bump at 45Hz and a slight dip at 50Hz. Aligning phase at 50Hz helped smooth the frequency response in the region of this wrinkle and helped preserved transient 'slam'.
4) carefully tuning the graphic, with an eye to fitting the graphic contour limitations between bands first. Since there's less tuning flexibility at intermediate frequencies, prioritizing the fit at those frequencies helps smooth the response. I ended up using more cuts than boosts on most channels including the subwoofers because some response dips aligned to the frequencies between graphic bands and the only way to use that fixed band characteristic optimally was to cut adjacent frequencies.
Some day I'll upgrade the system, but for now it suits my purposes. Given the difficulty of the acoustics, I'm not expecting much improvement to the LFE with more expensive gear unless I increase the subwoofer displacement substantially for extended frequency response below the first mode.
The bass response varies so drastically between the leftmost and rightmost listening positions that it hardly seems worth changing the gear at all. Even adding DSP to smooth the frequency response will only work at a single listening position. Room acoustics is the single most significant variable and all the money in the world sunk into gear won't fix a broken room.
Eventually I'd like DSP and BassEQ because the sub bass in new movies is really messed up, by design apparently (why?), but I'm also considering whether I can fix it with FFMPEG in preprocessing instead and not have to use DSP for bass EQ during playback. Some day when I'm bored I'll give it a shot.