You really want to stick with the same manufacturer (ideally even the same product line) for your front three to achieve proper "timbre matching" - that is, sounds that pan across the front sound stage are tonally consistent from speaker to speaker. Mixing and matching brands/models will often result in wildly inconsistent panning audio that will likely be just as distracting as your current low volume problem.
A quick online search reveals that when the MKII's were released, there WAS no "matching center" channel for them.
Therefore, the "best" match for the 7se mains would be another 7se for the center. In fact, audio purists would tell you that if you have the space, identical speakers all around are the best option for home theater.
Of course, this is not often a practical solution for most people.
I saw a pair of 7se's on eBay for a Buy it Now price of $165. If you have the space, it's a good deal and you get a spare speaker.
If that's not in the cards for you, I saw a few mentions online of a Paradigm CC-300 being a decent match, too - that's another speaker you'll have to scour the used market for if you want to find one.Go to the full post