Ok, to push the discussion a little further... In reality, you can design an 8 inch 2 way to be somewhere around 3db down at 35 hz and still go out to 2k to blend with a tweeter. Of course, every 8 cannot do this... Way back, I had a peerless woofer that had enough excursion that when adding enough mass, it could go comfortably down to the mid 20's in a sealed box, but it would loose its mid performance. We did use it occasionally as a sub, never developed or used it in big numbers. So, you can definitely, get an 8 inch woofer to go down. The big deal that has already been discussed is that as the driver gets larger, it can move a substantial amount of air. I have hear 15 inch 2 ways that I could live with, but didn't go down any lower than a decent 8, yet the difference in impact of the air moving along with the added sensitivity was a great experience.