Is it necessary that the center channel has to be the same brand as the mains? Or could I use a different brand?
Yes. Not only should it be the same brand, and not only should it be from the same model line, but it should* be the exact same speaker. Of course, that's usually not so easily accommodated. They should all ideally be vertically arrayed, all on the same plane, all at ear level. If there is any type of display involved, almost the only time you see this setup is with a PJ screen that is acoustically transparent.
Anyhoo, you have good speakers. You want a 340 center speaker. Since finding a "classic" 340 might be pretty tough, I'd consider the following options, in order:
1. See if your present 340s can be upgraded to SE easily (maybe it's just a xover upgrade?), and then add matching 340 center (which I'm guessing is less than $300 anyways).
2. Ask Ascend directly if they can make/modify their existing 340 SE center to be closer in signature to your present speakers. Maybe they have an original lying around anyways, I dunno.
3. Use a 340 SE for center speaker, and to hell with the slight mismatch.
