Why do you say that 20K is better than 10K? The difference is in their color. 6500K is the best for growth (of corals and algae), though 10K is more popular for replicating sunlight in the water (for coral growth) . . . 20K is more "blue". 1400K would be inbetween 10 and 20, and may be a better option if its going to be the only light you have (if thats available in 70W bulbs).
Will 20K 70W MH be enough to grow SPS in a BioCube? I don't know.