{1) There is a lot of talk about different designs, but where are the design specs for these folded horn subs available? Is there a listing of the most popular?
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Yes Wayne over at Pi speakers has what's been call "the Bass bin from hell" from a few friend's of mine, the 12pi, plans are availble for free.billfitzmaurice.com has plans but you have to pay.
Of course the Lab Sub on prosoundweb.net for free.
And if your across the big pond, speakerstore.nl has designs availble, the "Punisher" series seems pretty popular.
Also from across the pond speakerplans.com
{2) There is tons of discussion about the folded horns, but I haven't seen any about what you are using along with them. What is used for the mids and highs? Is it a line arrays, or something else?
The most efficent you can get preferably horn loaded or line array's
{ If it's a line array, do you have a specific recommendation on building one? }
Go over to the Line Array forum and ask for Jim Griffins white paper. For a newbie in sound I wouldn't recommend line array's off the bat, they can be a bitch sometimes.
HTH
Bill W.