It's a cliché that's worth repeating.
The Center speaker is the most important speaker in your system for Home Theater.
Look around the local retailers and web dealers for speaker systems. You'll find most 5.1 speaker systems build a center speaker that matches the other speakers in the system, about the same size drivers etc. Some speaker systems will give you the option of larger front speakers. For home theater this isn't necessary, although larger speakers with larger woofers can produce deeper bass the speaker should be set to "small" in the receiver. The small setting means that sounds below the crossover frequency will be diverted to the subwoofer. A typical crossover is 80Hz. So, the front speaker won't produce any of the lower bass instead those sounds will be diverted to the subwoofer. If you have a competent sub in your system the effect of setting your speakers to the "small" setting will produce more bass. We won't delve too much into bass management, that's another topic. Suffice it to say that for Home Theater it's a good idea to set all your speakers to "small" even if you like to switch it back for dedicated two channel (music) sources.
The center speaker arrived back in the days of Dolby Pro-Logic. Dolby Pro-Logic was the improvement on Dolby Surround that had a problem with hard to hear voices. By giving the midrange its own dedicated speaker dialogue in film could now compete with splashy sound effects and musical scores that dominated the front and rear speakers in a surround system.
The center speaker is responsible for reproducing sounds in the range from about 500 to 5000 Hz. This isn't a hard limitation of the center channel just an estimate of the midrange. In a 5.1 system the center channel can receive sounds from any frequency.
There is a lot going on in that center channel. Not just voices but a lot of sound effects too. Virtually anything that goes on front and center to the camera's perspective can be heard through the center channel. A frail center speaker will rob explosions, gunfire and all those other splashy effects of the bang they should have, despite the big sub and giant front left and right speakers.
One of the most common complaints when you have a center speaker that isn't up to the task is muted voices. Dialogue gets drowned out when the action or musical score gets loud. Chances are the center speaker isn't able to produce enough midrange or the midrange has some dispersion problems which can be associated with the speaker's build.
Sound Speaker Build Quality
One of the classic "cheap" center speaker builds is the MTM (midrange, tweeter, midrange) build. This is a two way design. One tweeter sits between two woofers. Since this design has no midrange it's already off to a bad start. To make an effective center channel speaker of this design is a challenge to the speaker's builder. The design opens you up to acoustic dispersion problems called the lobing effect.
Lobing is a bend in a circle. The circle of sound from your speakers should converge on the audience. But if your center speaker is exhibiting lobing it's difficult to accurately judge the sweet spot. The net effect is the midrange suffers.
Look for a center channel speaker that has a midrange and tweeter stacked on top of each other with woofers on either side. This is a three way design and isn't usually found in the most budget speakers. Most manufacturers only build their top end center channel speaker in the desired three way design.
Not to pick on Paradigm, a fantastic Canadian speaker manufacturer. But you must go well up Pardigm's product line to get into a three way center. If you can afford it you'll be glad you did. The mainstream line is the Monitor series, they're an excellent buy. But the CC-370 (pictured at the top) is an MTM design. I have not auditioned them or put them through the test. To really determine if they exhibit lobing problems you'd have to try it for yourself in your own livingroom. But I would automatically avoid that speaker when looking to spend a bit of money on a quality design.
To get the three way design you have to go two rungs up the Paradigm ladder to the CC-570. The 570 is part of Paradigm's "Reference" series of audiophile quality speakers. The 570 is an amazing speaker that will knock you back into your seat. The CC-370 is a $300US speaker where the CC-570 retails at almost a grand.
Think of it this way, a center speaker should include a midrange! A real center will make a dramatic difference for movies. For two channel music (CD, MP3 etc) it can only serve to fill in the central soundstage, not exactly a critical job if you have well positioned front speakers. But, if you've upgraded a two channel stereo to 5.1 surround for Home Theater and skimped on the center speaker reasoning you'll upgrade it later, you might be wondering why some soundtracks seem robbed of oomph and sometimes dialogue is a bit thin. Well, later is now.