The AD-900X is not bass-deficient at all. It sounds like your best bets are either to talk to /r/headphones about basshead headphones or to switch to speakers altogether. Running both at the same time gives you the disadvantages of both, a sound signature that will change significantly as you move around, and likely significant phasing issues.

Sound waves are basically a back-and-forth vibration of the air. It's a push-pull-push-pull. The wavelength of a 50hz tone is about 20ft. (roughly) That means that if two sources are 10ft apart, emitting their 50hz tone in the same direction at the same time, by the time the push from one reaches the other, the other will be pulling, and vice versa. They'll destroy each other's sound. (This is actually related to why active noise cancellation works.) The distance and effect will change with frequency, so some frequencies will be magnified, some diminished, and some just kind of smeared all over. Your body would feel more bass, but your ears would get shittier bass. (Not, of course, that the speakers and headphones would be guaranteed to be perfectly in sync to begin with.)

That said, if you want to try a software approach, a couple of free options are Voicemeeter Banana (yes, that's its actual name), which allows EQ and routing (i.e. it could turn up the bass for your headphones, but it could also function as a crossover if you really want to), or the combination of Equalizer APO (system EQ) with Peace GUI to configure it.