Hey phase - at the recommendation of Mr. Johnson at GMA, I got one from Marchand electronics. Nice guy, quick build time, high quality. Very flexible about building it to your specs to work best with your speakers.
I guess this depends on your definition of "affordable"
There are several "low cost" (<$500.00) professional audio crossovers. Do a web search for Rane or Behringer you'll find several instrument dealers that sell via internet.
I've used both Musicians Friend and Sweetwater and never had any trouble.
Does not fit the variable slope HP requirement but this looks interesting http://www.audioc.com/accessories1/rdes/rdes.htm A bit more discusson here http://www.audiocircle.com/circles/viewtopic.php?t=13650&highlight=
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