Interesting! We more often get folks who have a front pair and are looking for a matching center. So this is a bit of a reversal
The good news is, you got yourself a pretty darn good center speaker there! So that's a good start, at least
Ideally, what you're looking for across your front three speakers (Left, Center and Right) is a perfect
sonic match. You want that so that when a sound pans across the front, moving from speaker to speaker, there is no noticeable change in sound - it all just sounds like one, giant, continuous speaker!
This is easier said than done.
You don't necessarily have to use three identical speakers, three speakers from the same line, or even three speakers from the same brand! But typically, it is easier to find a good sonic match by sticking with the same brand and line of speakers - as they will typically use the same drivers (tweeters and woofers) in all of the speakers in that line.
That said, there are a lot of center speakers out there that are NOT a perfect sonic match to the rest of the speakers in the same line. They're usually meant to be. But making a horizontal speaker that sounds the same as the vertical speakers is not an easy thing to do! You can't just take a vertical speaker and flop it on its side. So, once again, easier said than done!
I can't say I've heard the C360 in person for any length of time or in any sort of proper audition. I do recall coming across them in a store setup at one point, but that's hardly any sort of proper evaluation. So I'm relying on the professional reviews that I can find, plus a bit of knowledge about how Infinity and the Harmon group tend to develop and design their speakers. In general, they rely upon and target the designs of Floyd E. Toole.
So first up, the obvious and easiest answer is that you should look for the matching Infinity Beta 20 bookshelf or Beta 40 tower speakers. The ES250 surrounds would be an option for the surround speakers. It's much less important that the surround speakers be a perfect match to the front three speakers as their primary job is ambience, not direct, precise sound. The ES250 are nice in that they give you plenty of options for their dispersion pattern. You can choose to run them as monopole, dipole or bipole speakers. So what ever sort of dispersion you prefer, the ES250 can accommodate.
If you're here asking though, I'm thinking that maybe you're not able to find the matching Beta series speakers in your area? If that's the case, then you'll want to look for speakers with similar design goals and an overall similar approach to give yourself the best chance at a good sonic match.
From all accounts that I've been able to read, the Paradigm Monitor series ought to be a fairly decent match to the Infinity Beta series. Paradigm's speakers are widely available, so you should be able to find them in your area if the Infinity Beta speakers are unavailable to you for some reason. Paradigm's speakers have similar design goals of neutral response and wide dispersion, which all stem from the same research into speaker design that Floyd Toole pioneered at the NRC in Canada.
So if the other Infinity Beta speakers are not a viable option to you, I'd suggest giving some Paradigm Monitor series speakers a try. For the surround speakers, it's less important to get the same brand and series of speaker. A very good all around surround speaker choice that tends to work well with virtually any system is the
Emotiva ERD-1 surround pair , which is on sale right now for a mere $250/pair. The ERD-1 is a great, neutral di/bi-pole surround speaker that plays well in virtually any setup. So at its current clearance price, it's a great way to save a bit of money and still get great surround performance
Hope that helps!
