One exception to atmasphere's comment directly above is a pre-amp using a
transformer to create the balanced output(s). That's exactly how Tim de
Paravicini does it in his EAR-Yoshino pre's. There is an EAR 868PL
(phono and line stages) listed on Audiogon at the moment, and it has
two pair of both XLR and RCA outputs.
Actually if a transformer is used, it has to have a switch so one side of its secondary can be tied to ground in case the output is to be single-ended. So a switch is required.
The only way to do balanced otherwise using a transformer and no switch is a a transformer with a center-tap. But this would sacrifice a lot of common mode rejection performance since the center tap can't be exactly placed at the center of the transformer windings. which is why no-one does it that way (as far as I know).