I have the NX1 too and I use the hardware acceleration on PD14 and my Nvidia GTX970 GPU. Your card GTX960 is also good for HWA. It renders my HEVC 2160p25 in just 1,8 real time ( custom profile at 80Mbps). The problem is that when I update the Nvidia drivers the HWA becomes disabled. Which driver do you use? I had v359 and reverted back to v358.91 and now the HWA works fine. I used v358.87 and previous drivers for months without problems but when the v359 was released, PD crashed.
I use PD14 almost daily. My videos are on YouTube rendered at XAVC-S 100Mbps and HWA.
Corsair Vengeance C70, Asus Geforce GTX 1060
Asus Prime Z370-A, Corsair RMx 850, Samsung SSD 960 EVO M.2. Windows 10,
Corsair Vengeance LP 2X16Gb DDR4 RAM,Intel i7-8700K