If on a tight budget, here is a possible solutioin: Don't think about biamping (a good crossover costs a few hundred anyway + the cost of the extra sub amp will be a couple hundred). Instead, try out a passively crosssed solution. I'd buy a pair of drivers from parts express (the pioneer foam surround woofers come to mind), put them in a BR box, and cross the entire thing 12 db / octave at around 70 Hz. Match for efficiencies, you may find that you have to lower the effs of the woofers (a first, eh?) to get a solution that meets your needs. You could build the BR box so it attaches to the back of the horn or something. I think this solution would also sound very nice.