The best thing is to start by buying a measurement mic, so that you can measure any speakers/subs you have now or will in the future.
This one is rated for 5hz to 24khz, and is professionally USA lab calibrated. Well worth every penny...
Once you have a mic, pretty much everyone uses REW for acoustical analysis, because it's detailed, and free:
One thing you will notice with concrete floors is that they don't vibrate, not even with 6 18's pumping full blast.
A lot of people build raised flooring, even if it is just a 2x4 laid sideways, or a riser for just the seats.
Wood vibrates a lot more than concrete, even when the subwoofer isn't touching it or even near it.
A lot of people place subwoofers firing directly into the back of their seats (nearfield), potentially even at head level. It seems silly, but you can eliminate almost all room-modes and gain like 100x louder bass by doing it this way. Which means saved money, smoother bass, instead of needing 16 18's and 8 amplifiers + massive bass EQ at 10ft away.
For corner bass traps, it is best to make them HUGE corner wedges (the thicker the better), and then cover the fronts with plywood that has holes drilled into it. This causes the bass to be trapped but more importantly, doesn't become a blackhole catcher of tweeter/mid frequencies. The plywood reflects the highs and the bass gets stuck behind the plywood. They cover the plywood with colored fabric to make it look nice.
It takes up to 10 to 40% wall coverage to reduce the clap echo you hear from bouncing off the walls/ceiling/floor and sounding nasty.
Anything above 50% coverage will start sounding dead, which you want to avoid.
A lot of people are using staggered treatments, as you only need to absorb the sound on one of the two parallel walls (50% cost savings.) Treating both sides of the walls in the same spots often is counter productive, needlessly expensive and isn't necessary unless it is required to maintain a good stereo image.
When nearfield subwoofers aren't possible, the next loudest spot would be either one sub in each of the 4 corners, or mid-wall points.
It is said that subs placed at the 1/4 wall points will have the smoothest bass, but often you'll lose a few db overall.
Keep in mind that subwoofers that are placed within 3ft of each other, act like a single subwoofer. Which basically causes them to increase the SPL by the full 6db. If they are spread on opposite sides of the room you might only gain 1-4db per doubling-up of subwoofers.
Obviously if you fill the room with subs, not only would it be loud from brute force, and smooth from multiple placements, but also loud because they'ed sum like a single subwoofer without even needing to apply delay/phase DSP.
But that is pretty impractical, because it costs a lot and consumes lots of floor space (unless we are talking 5 IB walls