I had the chance to do my first true fine tuning of my three subwoofers using miniDSP 2x4HD - two Rythmik L12s and one Rythmik F18. REW's auto-generated EQ did OK, but I wanted better results, so I started over.
I individually adjusted delay and phase inversion on each sub. This fixed a huge
portion of the peaks and valleys. Turns out I don't have a room null at 120Hz at my seat, I had a sub integration issue that this solved.
I then used REW's EQ to DSP the F18 alone since that drives a huge portion of the low bass. I applied a high pass filter on the L12s to limit them below 20 Hz (and to limit distortion). Finally, I applied DSP to the input signal using REW's EQ tool and some fine tuning on my part.
I forgot to save the REW runs after I truly finished (I know! kids were driving me crazy and I slipped) but this was from before the last round of fine tuning to get a bit smoother response.
Overall, super impressed with how powerful the miniDSP 2x4HD is. Highly recommend it for integrating and tuning subs.