Usually the "Acoustic Center) of the Speaker is between the Mid and Tweeter... for up close measurements... For seating distance, you usually want the Tweeter to be at ear height, or often within ±10º vertical.
I guess the first thing you should really do is confirm that the height is right.
After that, if you have gear for acoustic measurements (REW (freeware) and Umik (~$100), for example) you could take basic measurements and actually see what you are experiencing.
Experimenting, though, is easy and can be cheap or even free. Depending on amount of height needed, some wood blocks or bricks could help you out.
Important thing to keep in mind is that as you lean forward, you aren't just changing your ear height but your actual position in the room. I can attest that inches matter is such things. It is possible you are in a null spot for certain frequencies, which is why measurements can be helpful.
Anyway, experiment before spending!
