Which speakers would be the best choice

Put everything you can on the front and ditch the rear.

I have the Polk db 6500, previous version to 6501, and if it’s similar, (I think it is) than rule it out for any serious sound quality. It’s a good budget component set with good mid bass on 100 watts rms, but not for a true sound quality setup. For example, I swapped the Polk mid woofers for the Dayton Audio ND140-4 mid woofers I had and they sound like they have a wet towel over them by comparison and no amount of tuning with my DSP can help it. I’m using, or have used Focal, Dayton Audio, and SB Acoustics, and many others as reference.

Pre fabricated component speakers are tough because they are usually made of poor quality drivers, especially the tweeters for the money. Look at AudioFrogs GS line of speakers.