Polk makes decent stuff, they also make shit. So you really need to see if you can find the actual model number and search that. usually its on the back.
a real simple beginner trick is to check the cabinet. If it feels heavy and solid, there is a good chance its at least decent. if it is light and plastic, it likely is trash.
Quality wise WAV and FLAC will be exactly the same while FLAC allows your to tag and add metadata so I would suggest FLAC