Also, keep in mind that your Onkyo has Amps and can easily be used to power your rear channels. You could save $200 and buy and XPA-3, or if you go with the NAD, you could use all 5 channels up front and bi-amp your mains and use the other for the Center. However, I'd probably go with the XPA-3 or XPA-5 and forget bi-amping.
If you think you may possibly get a separate processor, like the UMC-1 down the road, than the XPA-5 would give you the best flexibility for the future....