Just a couple of points.
If you're going to set up your mains to run "full range", meaning that you have them set to "Large", then regardless of your crossover point, you will be getting bass from your mains AND your sub. Most speakers aren't very good at playing tones below 30Hz. I believe yours are rated to 32Hz or so. This is plenty low, but I guarantee you, you're not going to be getting appreciable output at 20Hz or lower. Plus, you're going to be stressing out your receiver, asking it to put out the power your mains will need to reproduce those frequencies. You're probably better off setting the mains to "small" and dropping your crossover to 60 or 40Hz.
Second, you mentioned getting another identical sub if you find that one isn't doing it for you. Remember that getting another sub just means that you will be moving more air. It will be louder, but it won't go any lower. So if you're looking to get deeper bass response, 2 of the same sub, if it doesn't go low enough, isn't going to help you.