There could be a number of issues taking place.
First, your speakers could be too small for your room. How big is your listening area?
Second, your speakers are not efficient enough to generate enough volume in your room.
Third, your amp is under-powered for your speakers in your room.
Since you do not want to replace the amp, more efficient speakers are an option, but possibly a costly solution.
Have you considered moving to a more of a nearfield listening position? The closer you get, the louder the system will be, I don't know how close you are right now, or if it's an option. If you can move the speakers and the listening chair, it is by far the cheapest option.
First, your speakers could be too small for your room. How big is your listening area?
Second, your speakers are not efficient enough to generate enough volume in your room.
Third, your amp is under-powered for your speakers in your room.
Since you do not want to replace the amp, more efficient speakers are an option, but possibly a costly solution.
Have you considered moving to a more of a nearfield listening position? The closer you get, the louder the system will be, I don't know how close you are right now, or if it's an option. If you can move the speakers and the listening chair, it is by far the cheapest option.