Bass is clearer and more impactful when I stand vs. when I sit

This also illustrates why placing speakers in a room often requires some degree of compromise  If full range speakers with no sub are used, the spot that generates the best bass response may not work well for imaging and vice versa.  And while a separate sub(s) may free up the placement of the main speakers, there is always that difficult issue of transitioning at the crossover point -- again placement that generates the best low bass may not be the optimum spot for the best transition at the crossover frequency.  

It takes lots of experimentation to find the best compromise for you in your room.

I improved the sound of my listening position by getting a different chair and changing the rake of the speakers.

If you are using as subwoofer? Like millercarbon and mlsstl said, experimenting with the position of both your listening spot and the speakers is the key. If you are using a subwoofer you can also experiment with positioning it's height by putting something under it. 

Nodes.