Building a simple three way is usually pretty forgiving, but as you've discovered, building a line array inexpensively without taking care to see that all the elements fit together like a glove can be less than successful. Its easily done, but you have to do a lot of design looking and waiting ahead of time.

If i were you I would simply replace the fosters with a three incher. But I designed my system so that the speakers would work together before I started. If you search around you might be able to come up with 32 sammi's and pay not more than $5 each. Here's some guy who will sell you a case for less than $3.99 each:

http://cgi.ebay.com/3-1-4-SAMMI-WOOFER-MIDS-CW77B10-4OHM-ALLISON-NEW_W0QQitemZ280129614660QQihZ018QQcategoryZ3276QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

I'd keep the electronic crossovers, though you could get one of the heavy hitters in passive's over at PE Tech form like Jeff or Pete or Curt to actually design a passive for you. But in my opinion, The evils of passives bring in more troubles than they are worth.

As I see it, if you go to a single tweeter you lose the low distortion and the wonderful line array benefits. But it all comes down to how much money you want to spend.

Compare 16 PT2 planars for $28 each for $448 or the B3n's for $305. I's stick with the domes if I were you. 64 domes are incredibly distortion free as well as super responsive. If you go with a tweeter you really need one that can give you the same sensitivity as a line of mids, but with individual volume controls on the amps that is not as necessary as when you have a passive crossover.

You might want to consider the benefits of putting the 3 inchers into individual tube enclosures also, but that is pretty labor intensive.

Just my thoughts. Others may have better choices.

Marlboro