Wow Polk provided a sub sensitivity spec, that's a surprise but what's it based on? Hopefully 2.83V/1m which is the standard. Here's a review with some measurements if you haven't seen it
https://hometheaterhifi.com/volume_12_2/polk-psw-505-subwoofer-4-2005.html. Not my favorite brand for subs, but that's one of Polk's better efforts and likely as good if not better than the ones with the Nakamichi soundbar.
Yes kHz is an abbrevation for thousands (or kilo) Hertz. Most humans can't hear past 20kHz....and quite a bit lower for many of us older guys
That sounds then like a system spec for range, and not just the subwoofer part of that Nakamichi system. The wireless frequency in giga Hertz isn't going to tell you anything about the performance of the sub itself, that's just the wireless carrier frequency. Max dB at what frequency? Not a very helpful spec, just indicates it's capable of loud.
What speakers do you have with the subs now? They may be just as good as the soundbar's....hard to know.
If you want better quality subwoofers, check out sub specialists like SVS, Hsu Research, Rythmik, Power Sound Audio. Maybe even DIY if you have the inclination/time/tools/skills.