yea he may be fine but remember my 2 sa-12's in 4cuft tuned to 28hz would only play down to 26hz with any kind of authority anything lower and no output only movement although it may wind up working out for him also there is just way to many factors to account forHe might be alright with what he has and the tuning is already 28Hz. That's lower than most people's tuning...even mine.
Btw OP...you want ultra low end subs, get yourself some Sundown Audio X series. They're made to play really low notes, better than most competition subs.