This has nothing to do with the gear and everything to do with the placement and setup. Especially if you have a subwoofer involved, there is no reason it shouldn’t be nice and full sounding, your room is not what I consider big at all.
This could be as simple as the subwoofer is out of phase, knocking out the rest of the sound. Or it could be that your simply sitting in a null (from what you discribe this is more than likely) and simply moving your seating position slightly will flesh out the sound.
Until you narrow down these possible problems don’t spend any money. No amount of money will solve a null/phase/timing/ cancellation issue.
You need to tell us more about the room it’s always the room.
edit: does the subwoofer have an auto sensing circuit? When at low volumes the sub might not be registering the input from the amp. Thus not turning the subwoofer on. A 12 volt cable will solve that problem
It's not lik a subwoofer is the only thing responsible for making the sound lean
So using a subwoofer sound is lean, not using a subwoofer the sound would not be lean?
Can sound a tad lean, in my system it didn't