You should get more output from a pair of SI BM MK V subs due to more cone area and more excursion capability. The 13TW5 has less output than the 12TW3 because of less excursion due to needing to fit in a very shallow 2x4 wall for home. The BM MK V has slightly more output than the 12TW3 but they are close.

Regarding amps, you could work with what you have but 1200 watts would get them closer to full output capability.