Inna, I agree that all things being equal, 8" would be much better for bass but my room constraints would just exacerbate those room nodes. I've learned my lesson from having too big a speaker. Again, ask my neighbors. :-)
As to a single driver, I'm with you on this. I used to have a pair of Hornshoppe Horns which satisfied me for quite awhile but that 4" single driver just couldn't plumb the nether regions to my satisfaction. That, and the lack of "meat on the bones."
I just feel that since all speakers are a compromise, one should get one that gets the highs, mids and upper bass as close to perfect as possible within the confines of one's space: just let the room fill in the lower end as best as possible unless you're fortunate enough to have a room large enough to use a good subwoofer.
All the best,
Nonoise