I have a bigger overengineered receiver and i'm not familiar (a.k.a wouldn't want to assume things) about your Onkyo model. The circuitry and distribution for 5 channels driven vs 7 channels driven on your receiver is a unknown (however, they share the same power supply for how you have it biamped). Your speakers appear to be quite sensitive. So, you wouldn't have to keep cranking the volume knob (depending on how far away from the speakers you sit, listening levels, etc). It would be safe to assume they are not the easiest to drive though. If you can't hear a difference between how you have it biamped vs no-biamp (i think you won't hear a difference in your speakers), why bother? BUT, IF you can hear a difference, feel free to keep it. There are a few fire filled threads about this out here about this kinda thing.
A active sub crossed over can take some load off your receiver. That active cooler i suggested (very important) can be your safety net when you're cranking it up for hours.