Matching sub brand to speaker brand isn't important at all. Good to see you have a NX-BAS-500 that is still kicking. The amp on those proved to be unreliable. I haven't heard the Polk HTS12, but that NXG sub is a really good performing budget sub. I had two at one time and was very impressed with them compared to some of the usual suspects from Polk, Bic and Klipsch. If anything, adding the Polk to the NXG might be an option, but I really see no real upgrade in replacing the NXG with the Polk. Or you could sell the NXG and get the Polk. It's likely that the amp will die on the NXG and they went belly up as a company.
Subs>RBH I-12e, I-12. HSU VTF-15H mk2. Monolith THX 12(x2), THX 10(x2). XTZ 1X12. SVS PB-1000(x2). Speakers>JBL Studio 270/235c/230/225c, Arena 130/125c. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5bs. Polk RTiA1/CSiA4. Other Audio>Sony MDR-Z7m2, MDR-V6, XB900n, XB700, XB32, XB31. Sennheiser HD58X, HD4.50, PC37X. Philips SHP9500. SIVGA SV004. HyperX Cloud PS4. TB Stealth300. LG FH6,RK8,RK7,PK5. HK Onyx Studio4(x2).