Inna, in regard to low frequencies, we're talking about feeling as much as hearing; that means moving a lot of air, which is a bad thing for apartment living. I think what you have is the best for your situation. Another 8 inch speaker might be more expensive and sound better, but wont help in regard to lower bass; that's a purely physical thing involving the volume of air that is moved on each excursion of the speaker.
In regard to Phusis post on pushing a rather limited amount of air violently; that achieves lower bass, but how does it blend with the rest of the music if it's a sub woofer?
Inna, I used a 12 inch woofer in my bedroom, and my son in the next bedroom complained, so I nixed the 12 inch woofer; lower bass will create more problems for you, than it will solve.
Enjoy the music.