Hey rabbit! The problem is that the phase response of the low frequencies will not match between the sealed and ported. The port is a mechanical amplifier, it reacts to the woofer movement and pressure wave inside the box before it starts making noise and this has a delay. Ported speaker's low frequencies will be time delayed and out of sync with the sealed woofer's LF. Depending on placement relative to listener, The two bass sounds could cancel or augment each other in places in the room, making peaks and dips caused by the speaker differences. Also, the bass will sound muddier and less detailed the lower it goes and the more the phase error increases. Many people say this doesn't matter because our hearing sensitivity at LF is not acute. I disagree.
For small woofers like 6-8" then ported benefits usually outweigh the drawbacks. But with larger woofers, 12"+ the scale starts to favor sealed for audiophile quality music. For thumpin dance music and home theater LFE, ported is usually preferred sound wise. Most good designers try to put it somewhere in the middle to make punchy but natural sounding bass compromise.
If the ported center's woofer is small and the mains woofers are large, then the delayed center's port will blur the upper bass of the mains. I think maybe juststuffing a sock into that center speaker's port, and then using your receiver to boost up it's bass a little might be an option for you. Just don't boost it too much, it's not meant to handle low bass. Ask Polk if it's OK to plug the port as it might be needed for cooling. Usually it's not a problem. Polks usually try to swing above their size, so they lean heavy on the porting, it is probably wide open standard alignment, so the phase error will be max and the loudness of the error will be max. But if you block the port the bass of the small woofer will roll off higher than the port did, so you will need to boost it to restore some of the oomph. This is important on a center channel to avoid male voices sounding wimpy.
The side speakers won't matter as much. I would just keep those ported as you have.
If your receiver has no bass boost for center channel, you could use a
linkwitz transform to boost it externally, but this is a bit extreme solution.