Yeah I'd be careful of building a one note wonder bud. 30-32 is a good tuning for music. I also think 6-10 is a HUGE freaking difference. 4 cubes can make or break a box lol. You're all over the place talking about 600 rms then talking about an X15. You need to nail down something. You really need to narrow it down. I think 6 cubes for a single 15 is pretty good size but you're going to need at least 1500 rms imho.
You should also include what you have electrical wise. If you can't run the amp your dead before you start,