Need help with two subs and external crossover....

Ok, I own two Final Sound subs that I run off my preamp. I no longer want to use the crossover built into the plate amps as they don't allow me to crossover low enough - below 40htz.

I will need to purchase an external crossover for my subs only which I can set to 30 htz and below. I don't want to spend more than $300 or so and would like to hear suggestions.