I like the Onkyo receivers, but basically there are only a few receivers on the market (at the highest end at that) that I would say might be worth your time biamping. For biamping to ultimately work, you need to have sufficient additional power to do it; in other words - external amplification with its own dedicated power supply.
A receiver's power supply is powering everything in the receiver at the same time, so in essence the speaker won't get a significant benefit because each channel is still being powered by the same single power supply. Yes they are hopefully discrete channels, but those channels are still fighting for their share of the total power. The 60x Onkyos are good, but (no offense) they aren't stout enough IMHO to provide sufficient current to do true biamping. It is a marketing feature to me and little more.
I have tried biamping and biwiring like this and found little benefit.