The SE Oppo did it for their dedicated stereo outputs.
Most DACs these days have eight single ended outputs (to cover the 7.1). Some gear that offers balanced outputs will use two DACs to cover all the channels, and use them in their balanced mode, which outputs an inverted signal from half the channels.
Some DACs offer a high quality (single ended) stereo mode that uses half the chip for one channel and half for the second. Those DACs usually also offer a mono, balanced mode so all eight channels are outputing the same signal or an inverse of it.
See what ESS shows how their dynamic range improves as more channels of the DAC are stacked:
http://www.esstech.com/index.php?p=products_DAC It doesn't do anything for the harmonic distortion, but at -120 dB (about 0.0001%) there's not much left to do (note: there's no way to preserve that low level of distortion or even the dynamic range through the rest of the output stage).
