Put that Sub on the chopping block.
Depending on your usage, there are some great values, especially if you act fast with Christmas sales. Regardless, I agree with Shady that the sub isn't going to do much for you. It is possible your speakers will sound better at 35Hz than that Sub.
That said, I would recommend Small with the XO at 80 as a starting point. Experiment up or down from there (try 70, 60 (no lower) and even 90, 100). Pick something to listen too that you know well that has some good low end as well as Mids and Highs. Listen, change, listen, change... look for the subtleties in Cymbals or female voice... whatever it is. Take a week or two to just listen and fiddle with it. Eventually you should start to make out some little differences as your ears adjust.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
My experience with setting different XOs between Sub and Speaker left me finding better Bass through a capable Sub, and more clarity and detail in Mids and Highs through the Speakers.
As Shady said, the Sub will take some of the load off the Amp Stage of your AVR, it will also take some of the power demand of the Speaker, allowing the Speaker operate more efficiently by focusing the energy available to it higher up.