I've used the Parts-Express plate-amps and they are a reasonably priced fix.They are mono units.Since you are using two subs-two are required.
Onkyo for subs:You will need a crossover.Maybe a Dahlquist LP-1 or the NHT X-1.The LP-1 is passive (capacitor) on top and active on the bottom.If you have two outputs on your pre,you might run the monitors full-range and use the LP-1s' bottom (variable freq/gain) to control/balance the subs.This will allow you to adjust the subs and not introduce more circuitry into the monitors.
Onkyo for subs:You will need a crossover.Maybe a Dahlquist LP-1 or the NHT X-1.The LP-1 is passive (capacitor) on top and active on the bottom.If you have two outputs on your pre,you might run the monitors full-range and use the LP-1s' bottom (variable freq/gain) to control/balance the subs.This will allow you to adjust the subs and not introduce more circuitry into the monitors.