I have a Dayton RS 18" tuned to 18hz, and 9 cheap JBL 12s. All diy and EQ'ed flat. At reasonable to a little loud volumes it sounds great. If I try to 'show off', like make your eyes blurry, I haven't figured out how to tame the bass yet. I don't know if it's box resonance, room resonance or what. Since I rarely show off it hasn't been a big deal, but it does kind of suck when I'm like "check out what it can do!".. sound like butt, that's what. The clipping lights on the amp aren't going off and the subs aren't moving a ton.. anyways, those 5 minutes a year are just disappointing.
I bet your bass issue is easily fixable.
1. Flat bass is bad. Human hearing naturally prefers a downward trending slope, response at 25Hz should be at least 5dB louder than at 200Hz. I have an 8.5dB slope implemented.
2. Make sure your subs are properly located throughout the room. If you have 9 drivers located close together that defeats the purpose of multi-sub, which is balanced room interaction.
3. Check the distortion and decay times of your subs with a cheap measurement mic. Often you can find things like poor cabinet bracing, manufacturers lying about xmax, or just outright poor cone stiffness.