Also, for speakers that end up in the attic, can things be improved
by
building an enclosure behind them?

Or you can use a speaker with a ported and tuned enclosure. Dayton
makes those too:
http://www.amazon.com/exec/obidos/AS...rkthomasorg-20
Has a larger 9-1/2" cutout, but would be good for attic-above areas.