Hi, Gary. I agree with Mark, so I didn't add anything. But, I'll expand a bit here.
That Polk is certainly a decent sub, especially for the money. At loud volumes and/or low frequencies, though, it's not going to be as clean as a more capable sub. So, a more capable sub (i.e. more expensive in this case) might help - but the muddiness might also be due to room interactions.
There are a few things that you can try (and hopefully you know where in a given movie this muddiness occurs so you can play that scene over and over):
1. Set up your receiver to not use the sub. Set all of your speakers to Large and set the sub to Off. See what you think. Does it sound better to you? If so, maybe you don't even need a sub.
2. Change the crossover frequency from 80 to 60 Hz. That's not ideal given your speakers, but it's a test to see if either the sub isn't handling those upper frequencies very well or if it has some interactions with the room there. It's a step between not using a sub at all and using it how you are now.
3. Like Mark suggested, experiment with the location of the sub. Audioholics as some good articles on
subwoofer setup including at least one on
subwoofer placement. Adjusting it just a bit can help out with how it sounds at your listening position.
