As great as the 340SE2 are, our Sierra line is a step up in performance.
Yes, you can use our Horizon with any of our other speakers, no issues there. That stated, first step is to decide on your mains. If you are going with 340SE2, than I would suggest 340SE2 center. If you are leaning towards Sierra-2EXV2 or Sierra-LX, I would recommend going with Sierra-2EXV2 center or LX center.
Please feel free to email me directly.