I had the ESL C, wasn't bad but wasn't the most impressive center channel I've had, I had a 50xt before that which i found was much better with both my ESLs and Theos at the time. I would see if any stores have one in your area for a demo as well as any other Martin Logan Centers. You probably would be content with the ESL-C as it will blend in better but I think it will also leaving itching for more.
For an electrostatic center the C18 is lightyears better and overall one of the best sounding centers I've heard, but it is pricey. I think would pair better with your Ascents if your budget allows it.