Craig Chase
Gear Guru
No matter what speaker you use, it won't be flat out of the box. That's because the room has as much of an impact on the response as the speakers do. In theory, any sub capable of producing sound loud at all frequencies can be made flat through equalization .It's easier to pull things back be EQing subtractively than it is to boost dips.
I have a 12 inch Sunfire True Sub designed by Bob Carver. I don't think they make them any more, but Sunfires have huge power amplifiers that are designed to produce a lot of sound in a very small footprint. The way I calibrated my system was to listen to music that had descending bass patterns (Reiner's Marche Slav on Living Stereo is perfect for this) and calibrating the crossover by ear to try to balance all the notes as they descend. There is a Beatles song on the White Album that does the same thing, but right now I forget which song it is. When you EQ, you also need to balance the bass to the midrange and the treble, so you have to work the whole response at once in passes. The ear is more sensitive to imbalances at loud volumes and spikes need to be worked out progressively, so it's best to start at a low volume and work your way up. Relative volume of the six channels can make a huge difference too, so you are balancing two things that affect each other at once. That takes time and parallel parking to get right.
When I had the entire audible spectrum on each of the 6 channels the way I wanted it, (it took about 2 months of listening and adjusting to get there) I called in a friend who is a professional sound mixer. His job is to EQ concert and club sound systems. He ran tone sweeps and checked to see if my ballpark EQ was correct. I only have a 5 band parametric equalizer, so my ability to do notch filtering on very narrow imbalances is limited,. But he verified that with just a small 2-3dB adjustment in the upper mids, I was as close as I could possibly get with my current equipment.
The nice thing about subs is that the lowest octave is felt more than it is heard, so there's a lot more leeway there. The hardest thing to achieve is an even crossover at 80Hz from the mains to the sub and down to 40Hz. That's why I said before that the sub bass that shakes the walls is easy. The hard part is integrating the sub with the mains to make the bass smooth from top down.
Hope this helps.
Besides the Sunfire, what other subs have you calibrated? Do you have any response curves that show the lower limits of your subwoofer in your room?