In absolute terms, the speakers are pretty much always going to be the weak link in any audio system. You, however, seem to have wisely chosen better than most people in how you have allocated your funds between speakers and the receiver/amplification, as most people cheap out on the speakers too much for how much they blow on the receiver/amplification.
But, of course, if it is not set up properly, that can make a huge difference. And that is where I would try to change things in your case.
Assuming that there are no nasty low dips in the impedance of the speakers (or in other words, assuming that they are an honest 4 ohm speaker), that receiver should be okay with those speakers. You will not be able to drive them to their maximum volume, but it should be fine if you don't listen at deafening levels. And if you do want to listen to higher levels than you can at present, you can buy a separate power amp and use your particular receiver as a preamp/tuner, as it has preamp outputs for that purpose.
Probably the best thing you could do to improve the sound is to change how your system is setup (i.e., move it to a different room, or reposition it and/or other things in your room).
Normally, you want to form something close to an imaginary equilateral triangle with the speakers at two corners and the listening position at the third corner (though for some speakers, this is not right, so I recommend that you look at whatever advice is in your speaker owner's manual for your specific speakers). You should probably either be closer to your speakers or they should probably be further apart. And you probably don't want a glass bookcase between them. You might want to simply pull them further out into the room, as most speakers don't sound their best up near a wall anyway (though, again, consult your manual for your specific case, as some speakers are designed to be up against a wall).
Probably, though, it is best to start as j_garcia did in his response to you, in asking what the specific concerns are that you have about the way it sounds now. What is it about the sound that displeases you?