It has already been mentioned countless times that one should use a speaker with the same drivers as the front pair, or at least the same tweeter as the fronts.
As for how to place it, some basic principles:
- it should be on a stable, non-resonant surface. That means that if you are using some Ikea console, or a sonorous Queenie glass / metal equipment rack, you are not doing yourself any favors. You may think that this rack is on spikes, but the SHELF isn't. And usually it is made of MDF, glass or other potentially resonant material.
- use some form of isolation - that means you can place it on something which de-couples it from the surface that can resonate. Budget options include a pair of rubber door-stops, or those $2 blue pads from Daiso. More expensive and highly effective options will be those Auralex MoPads. I have already recommended them to a few other friends, and they have used them with great success.
- direct the centre towards your ears, this will enhance the illusion that the sound is coming from the centre of your screen.
- the topic of using two centres comes up rather often. In this case Two is NOT better than one. Google up "lobing" and you can read all you want on it. In essence, INTERFERENCE
- how about vertical vs horizontal?
- actually the ideal situation is to use the same speaker as the front pair. Even if we use the same drivers as the fronts, by virtue of the horizontal position, or the design (often even if the same drivers are used, the crossover design is different). However due to space, aesthetics or other reasons, centre speakers are often horizontal with a single tweeter and two woofer cones.
Try using another bookshelf of the same design as the fronts if you can. Or like the HTM4s of the B/W 800 series, it is the same speaker as the 805s but laid horizontally but with the tweeter one top. Another solution is offered by KEF, where their speakers are co-axial. That IMO, offers a truly solid speaker design. But it will still something not offer the same sound, simply because the speaker is not in the same box, the position is compromised.
However these days with the advent of auto-eq, small differences in tone can be ameliorated and you can get a seamless front soundstage.