Affordable external crossover for sub use.....

Check out the "Richter Scale" by Audio Control. Great piece for the money and will let you dial in any sub for room acoustics, etc. Although primarily designed to EQ subs, you can order it with any crossover characteristics you require for your system and/or change them later on fairly easily. If you buy one used make sure it includes the microphone (included if bought new) to enable you to dial in your sub with this unit's built-in warble tones.

Later, John Z.

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=