back when i was running the solo x 18, i was running it in 12ft^3 tuned at 22hz, and passband was acceptable. i was also running 8k rms to it, and never had any problems. its all in the enclosure, once that is right, your thermal and mechanical power handling will be where it needs to be.
I absolutly agree, but im a rookie box designer, and an OK wood worker, I am a carpenter by trade.
but yeah before i got these two solo x's I had just 1 on the full 10k, I wont lie it took probly 9-10k for a good 10-15 minutes before it popped.
But my box probly wasnt designed as well as yours.