This is mind blowing! Nobody talks about this in the home theater forums!
Yeah it's weird. I feel like the way people choose speakers for HT is fundamentally broken. You should make sure you're getting a good center first -- that's THE most important thing since all the audio comes from that speaker. Then worry about the rest of the speakers afterwards.
Are any of the horizontal centers good options though?
Yes, some are fine. The rule of thumb is basically that 2-way MTM(midwoofer-tweeter-midwoofer) centers are always pretty bad. Basic 3-way centers like the Emotiva C1+ or
Monoprice THX-365C are usually good.
The Emotiva C2+ is a weird one since it has two midranges and no one has measured it. I don't know if it's good or not.
The best case is a coaxial speaker on its side, which will typically be the same as upright(because a coaxial has the midrange and tweeter exactly on top of eachother). So Kef etc.
Now, can you mix different brand centers and L/R? IMO yes you can, but you need measurements of both for a good chance of success. The biggest risk is if the dispersion patterns are different -- there's research showing people really dislike this. So I would never mix a coaxial with non-coaxials for example. Also you should definitely be using room correction like Audyssey so that it will EQ them all to the same target curve at least. That won't guarantee they'll sound the same, but it will help. And yeah, no guarantees, you may not like it, it's not an easy approach.
Isn't the center channel used almost exclusively for dialog, which has a narrow frequency range.
That's a misconception. The center channel carries most of the audio in any multi-channel content. This includes dialogue but it's also the on-screen
foley and central vocals for music, especially if upmixing. It really needs to have the same capability as your L/R.
Anyway, the problem with 2-way centers is their
nasty, narrow horizontal dispersion, not necessarily the on-axis frequency response.