Are there any balanced electronic X-over's?

The Bryston 10B comes in a balanced version.

Pass Labs has a very functional and affordable balanced/SE unit called the XRV. Nelson Pass has been doing active crossovers for a long time. I believe all the Threshold units were from his mind. Here is the link:

http://passlabs.com/prodlit/xvr1lit.htm

Probably for less money you could get a Marchand just the way you like it. My XM9 2-way (balanced only, 250Hz, 24 db) is sold (so far). The frequency modules are very cheap, if you want to experiment. Go on their site (marchandelec.com) and check them out. I'm enjoying my XM44-3 which came single ended AND balanced. A nice touch.

Krell also made one for a while called the KBX..It's hard to find...