Ken,
Yes, old DPL and Dolby Surround is a matrix-based system, with the additional channels flipped in phase and piggybacked on the L and R signals. The center was just L + R and surround was L - R. The separation between channels was only about 3 to 6 dB, enough to make it sort of work. But for music it was hopeless. It seemed to collapse everything into the center channel.
DPLII still looks for out-of-phase material in an L/R stereo signal but operating in the digital domain, it's much cleaner and the separation is way better.
It seems to me the Dolby Digital decoder in the Sony set should do a quasi mix-down of the the 5.1 signals suitable for the L and R analog outputs, in which case there should be information there that the Kenwood DPL would extract. . .
Alan
