You say you don't want to add a subwoofer but in the next breath you want to improve "especially the bass".
A subwoofer or better yet, a pair of subwoofers will give you the most flexability for the quest of better bass. The biggest advantage being able to move them around the room to the best location for the least amount of cancellation. They also have the advantage of adjusting the phase which has a huge effect on bass frequency cancellation.
Subwoofers give you far more flexibility to properly tune the bass frequency than having the restriction of floor standers in a set location.
A subwoofer or better yet, a pair of subwoofers will give you the most flexability for the quest of better bass. The biggest advantage being able to move them around the room to the best location for the least amount of cancellation. They also have the advantage of adjusting the phase which has a huge effect on bass frequency cancellation.
Subwoofers give you far more flexibility to properly tune the bass frequency than having the restriction of floor standers in a set location.