the dual 2234H in the 4435 will be quite similar to one 18" in the bass department, and the box size will be similar (between 250 and 280L).
If you go for a two ways with 18" you will need a horn that can go down to around 300Hz, and that will take even more space than the bass box itself. Plus a horn driver able to go that low will likely need a tweeter...
If you want a 18" you have to go 3 or 4 ways (think 4345).
If you like the sound of your 4430 but wants more bass then the 4435 is the anwser.
Here is an alternative box shape with a smaller footprint:
http://audioheritage.org/vbulletin/s...ad.php?t=18329
http://audioheritage.org/vbulletin/s...ad.php?t=21536
contact member Guido