Well, you could attempt to use analog outputs from the TV and put the Kube there. Alternatively, I did notice this receiver has some very limited EQ (glorified tone controls) which you might be able to use to fill in.
One other option is to use a receiver with full room correction, which will have better ability to adjust the receiver for the lack of a Kube.
One other option is to use a receiver with full room correction, which will have better ability to adjust the receiver for the lack of a Kube.