fredk hit the nail on the head. I really do not think that you have any equipment short-comings. From what you described, it seems to me that you likely have room acoustics issues.
People often go on the hunt and spend lots of money on new speakers, new amplifiers, new cables, etc. All in an attempt to get better sound. In the end, everyone eventually realizes that good audio is an equal partnership between the gear and the room.
There is a very good possibility that simply moving your seat will result in large improvements to your sound. Many people have their seat pushed right up against the back wall. Or else they happen to be sitting in a bass null, where it just so happens that the echoes from the room's walls are creating a cancellation of sound.
So - ways to improve your room's acoustics:
1) Focus on proper placement. Try to place your seat so that it is 1/3rd of the way into the room. In other words, if your room is 15 feet long (from the front wall to the back wall), place your seat so that your head will be 5 feet from the back wall and 10 feet from the front wall. Also try to place your primary seat so that it is basically centred width-wise (the actual "ideal" is to be 6" to either side of dead centre width-wise).
Place your front speakers so that they are at least 2 feet away from any wall. And place them so that they are not exactly the same distance from the front wall and the side wall. In other words, do not place them so that they are 2 feet away from the front wall and 2 feet away from the side wall. Make those distances different - 2 feet away from the front wall, 3 feet away from the side wall, for example.
Do the "subwoofer crawl" to find the best location for your subwoofer. Move your seat out of the way and put your sub in the location where your primary seat would be. Put on some bass test tones (
this test tone CD is a great tool). Now, go to the various spots in your room where you would be able to put the subwoofer. These spots will likely be along the walls, but simply check the sound at each location where you could reasonably put the sub. Squat, kneel or crawl so that your head is at approximately the same height as when you would be seated. Pick out the spot where the bass sounds clear, even and undistorted. Once you find that spot in your room, put your subwoofer there, move your seat back into position, sit in your primary seat and enjoy the better bass
Getting a second subwoofer can greatly help to make the bass response even and flat across all of your seats - not just your one, primary seat. If you get a second subwoofer, the best spot for it is typically symmetrically across from the first subwoofer.
Decoupling your subwoofer is also very important if you want to avoid hearing distortion created by your room. All subwoofers physically shake. And when they do, the floor also physically shakes along with them. When the floor shakes, so do the walls and the ceiling. And this is why you can hear the bass throughout the entire house, even if the higher frequencies are inaudible! What you need to do to avoid this is to physically separate the subwoofer from the floor, and this is called, "decoupling". Essentially, you are placing a "shock absorber" in between the subwoofer and the floor so that the physical shaking of the subwoofer never reaches the floor beneath it. You will still get some room shaking, purely from the sound output. But it will be much less severe than when the subwoofer is coupled to the floor and it will only happen at the few resonant frequencies of the room, rather than at virtually all frequencies.
I highly recommend the Auralex SubDude or Auralex GRAMMA for a decoupling device. They are extremely effective. There are alternatives though that aren't
quite as effective, but still help a great deal, such as those foam sanding blocks that you can easily buy at Home Depot for just a few dollars
2) With placement out of the way, you can further improve your acoustics with passive room treatments. I subscribe to the idea that you want the sound coming from the front of the room to be very direct and free from reflections. I also subscribe to the idea that you want the sound coming from the front to "wash over" you and then "disappear" behind you. Meanwhile, you want the sounds coming from beside you and behind you (your surround speakers) to be diffuse.
So I subscribe to the fairly common practice of using absorption panels on the front wall and absorption panels on the side walls at the first (and sometimes second) reflection points as well as absorption panels directly behind you. On the ceiling and on the side walls directly beside you and behind you, I recommend diffusion panels. And, naturally, I recommend bass traps in the corners. You can also run bass traps along the wall-to-ceiling and wall-to-floor edges if you want to control the bass even more effectively.
3) Finally, after placement and room treatments have been tended to, you can use active equalization (EQ) to really dial in the final frequency response and get nearly ruler flat response at your listening position. Personally, I do not see active EQ as a "cure all" or a means of making a bad acoustic environment into a good one. Rather, I see active EQ as a means to get that last, tiny little bit of performance out of your system after you have taken all the steps above to make your room and placement as good an acoustical environment as possible
