I'd imagine it to be a sideways step.
If you are suffering sloppy music bass from a servo controlled sealed sub, have you considered that some sort of bass EQ might improve things drastically?
It's not guaranteed, but with your 3.5m dimension repeating in the 7m dimension of your room you'll potentially be getting a peak in the response at 25Hz along the length of the room, with a potentially larger peak at 50Hz across the room as the second harmonic of the length ways mode combines with the fundamental of the shorter dimension.
It's no surprise that the 50Hz mode could make most musical bass sound 'ploddy' and tuneless with a very one not quality and that's before you also consider the other harmonics of both the modes at 75 & 100Hz and then start looking at the tangential modes at 55 & 110Hz....
Baffled? Well I've prattled on for the fun of it but there's a very handy room mode calculator
here that helps you visualize whats going on. Where you spy a very similar frequency repeating, that can double your trouble. Type one dimension in at a time to see which frequencies relate to which dimension.
It's not guaranteed that all of the frequencies will be peaks as that calculator can't predict variations caused by where your sub is and where you sit - some of them could equally cause dips (a suck-out) in the response too. But if you have the de rigeur Radio Shack SPL meter and some freely downloadable test tones burned to a CD, you can measure your rooms actual response and see where the trouble really lies and whether EQ will help.
The good news is that EQ devices can cost* less than the change of sub you are proposing.
Russell
*I'm excluding your sanity from this equation.
