Simply adding another subwoofer to a system doesn't always make it better. In fact, adding two different subwoofer models (which each have their own frequency responses) with the intention of using them to "double up" the same LFE signal will often create a worse sonic situation.
The facts of the matter are these:
- Even though the 9400 is about the best "all-in-one" system out there, the speakers and the subwoofer are still the weak link.
- The Polk subwoofer is an average performer. For the same ~$100 you can do better.
- From the sounds of it, you haven't even purchased the 9400 system yet, so how do you even know that the stock subwoofer won't suffice in your room?
I would suggest holding off on getting another subwoofer - at least until after you have a chance to listen to the 9400 system in your room so that you can definitively answer the question "What needs to improve with this system?"
A smarter way to use two different subwoofers is to use one as a "mid bass module" that handles "upper" lower frequencies - basically helps bridge the gap between a "real" subwoofer and your mains. The stock 9400 subwoofer would suit this need just fine. Use the second subwoofer to handle the REAL lower end stuff. For this, though, you need a GOOD subwoofer for the low stuff. The Polk won't cut it.
Get the 9400 system, see how it works for you. If you still feel you're missing some low end, come back with a budget (hopefully $200 or more) and we can point you to a solid performing subwoofer that will truly enhance your system.