Descent I do not use downloadable custom crossover, that feature began with Balanced Force models.
You can run Descent I without the internal CLX cross-over card but it is preferred (it integrates the the subwoofer with the low frequency roll off of the CLX).
Connections are as described in the posts above, personally I use RCA O/P from my preamp and the balanced O/P from the preamp feeds my mono blocks.