Hi Zola:
I have written variations of this response over two dozen times, so here goes two dozen and one:
1. Do not spend a ton of money on speakers that will sit on an open shelf near a back wall or on an enclosed shelf inside of a bookcase near the bookcase's rear walls.
2. Use speakers that are either acoustic suspension/sealed box or front ported in design.
3. You will be making some compromises sonically by placing your speakers on a shelf inside of a entertainment center. In essence, you will have a box inside of a box and speaker companies really do not design their speakers to work optimally in this type of situation ... though in reality, it is probably a more common set-up than they might imagine. Typical audiophile speaker concerns, such as ... imaging; soundstage; separation; ability to disappear ... are difficult to come by in this type of set-up.
With this in mind, there are some things that you can do to make the best of this type of set-up.
If the speakers are more than 12" or 13" in height, you may have to place them on their sides. You can experiment with placement ... whether to place the speakers standing right side up or on their sides. With side placement, you can also decide whether you get better separation and dispersion with woofer in/tweeter out [o O] [O o] or vice versa speaker placement [O o] [o O] . If possible, try to place the speakers on a shelf that is close to ear level height (or even slightly higher) when you would be sitting down.
4. With all that said, I have had the best success with the NHT SB series (I own the SB2; $400 list) used with a solid state amp (NAD C 320 BEE) and also with the Omega Super 3/narrow cabinet $540 list used wth a tube amp (Prima Luna Prologue Two). I have had the least success with B&W 300 series (rear ported) and EPOS 11 and ESL3.
5. The NHT SB series are stunning for the money ... you do not have to pay a cent more.
Regards, Rich