I'm not an audiophile, but I have a very simple answer. It's the way they're made. Most Polk speakers have a down-firing port that minimizes reflections. And they have a classic brown walnut veneer. And they come with a 5-year warranty. So how do you fix this problem?
The T15 bookshelf speakers are the least expensive Polk Audio speakers. They have good bass, balanced mids, and decent highs. The sound quality is average for the price. There's no need to spend a lot of money on them just because they're cheap. Just make sure to compare them to comparable speakers from other manufacturers. You'll find the most affordable Polk Audio speakers on sale online. You can also find a review of them on Amazon.
There's also a chance that your Polk Audio soundbar is not set up properly. It may have a manufacturing defect or pre-existing hardware damage. If this is the case, you can return the speaker for a refund or replacement. If your soundbar is not the latest model, it may be time to contact the company and get a technician to test it. If the problem persists, you can also call them for assistance.