power handling in relation to box size

if i increase the size of the ported box...larger than the manufactures size...by 2 or 3 times, how will this affect the speaker if im still providing it the suggested rms.

example:

suggested enclosure: 2 cubes @ 36hz

suggested power handling: 1200wrms

actual enclosure: 6 cubes @ 36hz

actual power handling: 1200wrms

will this overpower the sub, and blow it; or will it underpower it, and i just wont get the subs full potential?