FWIW most high end audio preamps, even though actually balanced, don't support the balanced standard, known as AES48. AES48 prevents ground loops from making buzz, and prevents interconnect cables from having artifact (if you've ever heard differences between two cables that's what I'm talking about). This also means you can run the interconnect cables longer distances without a downside, and the cables don't have to be expensive to sound as good as the most expensive cables out there. Imagine spending the money for a balanced preamp and then finding out that its sensitive to long interconnect cables! That really shouldn't happen.
I run 30 feet of interconnect cable between my amps and preamp and it probably cost about $300 or so for the pair. So balanced operation is a nice benefit on this account alone.
If the preamp has a pair of coupling capacitors at the output, it doesn't support the standard. This is because the kind of circuitry that needs a coupling capacitor isn't able to tick the boxes (in a nutshell, its relying on a ground connection to complete the circuit, and balanced operation ignores ground entirely). The preamp will have either an output transformer for driving the balanced line, or it will have a direct-coupled output.
