Hey John, I went through similar issues with my Oculus and seat shakers. The game I was playing was Elite: Dangerous and never even tried DCS at the time, but the problem was the same.
The solution that everyone suggests is the VoiceMeeter app to mix Windows audio to different outputs. I COULD NOT GET THAT TO WORK. I never got anything to work with Oculus mirroring. Maybe you can have better luck than I did, apparently that works for a lot of people. I took the headphones off the HMD, hooked up surround speakers, and played that way.
Ultimately the only fix I could find was to move to SteamVR/Vive. Works perfectly there. I know "buy another VR system" is not an acceptable answer.
Yesterday I ran up an app call SimShaker. It runs in the background and sends tailored bass tracks (one for afterburner, gear up/down, etc) to your desired sound output. Works independantly of mirroring.
Ultimately that might be your best bet and cheapest option. I didn't need it to get around the mirroring since I'm on Vive now, but this is a hands-down improvement either way.
For example: The F15 sounded great to me when just running my shakers off the regular sound channel. Nice thumps for each stage of the afterburner. A10C thumped great when going over seams in the apron. Then I jumped in the Su-33. Nothing. Not all planes are equal apparently.
SimShaker fills all of that in, without any mirroring/mixing baloney. Hope it helps!