Yeah, you're on the right track. Setting the crossover on your sub to the max, prob some where around 120 - 180 should help. The only other advice I have for you is, regardless what the audyssey reccomends, try setting the crossover on the 505 as high as it will go, sending the most signal to your sub. Then you will be able to decide on between the two settings. Also make sure you run the test when you have your sub on a volume level which you plan on leaving it at, perhaps around 50%. I believe it's fairly easy to navgate through the menus, despite not having on screen capabilities.
If you are ever looking to upgrade your speakers I had great results pairing the 505 with a set of Paradigm Cinema 110's. It gave my apartment all the sound I could ever want. Since then I have been using the Paradigms with a Yamaha rx-v663, and an Outlaw Audio LFM-1 plus.
I really liked my Oknyo, I got good sound for the price, and when I first bought it, I too was using Sony htib speakers with it, so have a pretty good idea what you are dealing with. It actually is still working fine, my father uses it for his main HT setup, and has no complaints what so ever. He actually uses it with my sony speakers and a Paradigm sub, so he has basically the same setup as you do. Sorry if that was a little long winded, but I hope everything works out, if you need any more advice, feel free to ask.