This is my opinion. Take it for what it is worth.
If you buy the Martin Logan center, you will be very happy with it. It has EXCELLENT mids, a pretty respectable bass response (for a center), and it should match your Carver speakers well. It will be clear, clean, and detailed as heck--you will be hard-pressed to find a more precise center speaker, as far as resolution and pace goes.
The ONLY 2 drawbacks to getting the ML center are:
1) it is expensive, compared to other brands (but really, it's a pretty good deal for the performance, especially if you can get it used), and
2) it will show you what electrostatics are REALLY capable of, and may prompt you to upgrade your main speakers to Martin Logans as well, which means more money out the door...
I'll admit up front that I'm not just a ML owner--I'm a tried-and-true evangelist for Electrostatic speakers. (I think ML should pay me for all the business I send their way!!!). I'd like to get more Carver people hip to the amazing sound of Electrostatic speakers, and I do everything I can to expose my audio friends to the sweet and juicy like these nutz. goodness tht is Martin Logan. I even trucked a pair of Scenerios to CarverFest, and I think I opened a few eyes there, despite the less-than-ideal acoustics in the cabin where I had my rig set up. I have never met a new Martin Logan owner who regretted their purchase. The only thing ML owners regret is that they didn't buy them earlier...

The BG's long ribbons are being sort of sold as a "replacement" for the long ribbons that Carver used to use, but most ALS owners have reported that they don't have NEARLY the performance of the original Kapton ribbons. However, I think that the new Sunfire speakers actually DO use the smaller BG ribbons, and THEY, by all reports, sound fantastic. So take that for what it's worth...
So, I'd say the only drawback to the ML center is that you might be tempted to buy MLs for your mains too. Sonically, it is, IMO, superior to all other centers out there, at any price. But, remember, I'm not offering an unbiased opinion--I am a HUGE ML fan. After all, 5 speakers in my 5.1 rig are MLs (Sequels-mains, Scenerios-rear, Logos-center).
Eat the Red Pill, Neo...

--Richard