No expert, but I have exactly the same setup as you, except my iNuke is the 1000DSP.
I'd be surprised if you had
two dud Buttkickers. This sort of thing has often happened to me with audio, and it is usually just some setting or configuration I've overlooked or got wrong.
Do you have any tactile software ( e.g. SimVibe or SimShaker Wheels) or are you just using audio? If you are only using audio, is it possible that the frequencies coming from your audio are too high for the Buttkickers to respond to?
Try testing with the
Online Tone Generator, using frequencies between say 10hz and 60hz. Make sure it is mapped to your soundcard.
I think you can get a limited period free trial with Simshaker Wheels. It might be worth downloading it and seeing if that works.
Are you using the Xonar DGX soundcard for everything? See my Windows Sound screenshot. I have Oculus Rift's own soundcard as Default for sound and the separate Xonar DGX for the tactile/Buttkickers. My onboard soundcard is not used, but if I didn't have the Oculus Rift, the onboard soundcard would be the Default, for audio.
Here are screenshots of my settings.