Spray a little switch cleaner into the (mix) insert points on the Power Station and see if this has an effect on the intermittent fault.
If that solves the problem then the Mono/Sub out can feed one side of the amp to drive the sub (I read there's a crossover on the sub so that'll take care of filtering duty).
If it doesn't solve it then there's another way to creatively rewire the system but it'll need time, coffee and pen and paper before I commit myself.
Dear Mr God,
We called but you were out - B Dylan Deliveries (Intntl)