DB-25 is what Rotel used for multi channel connectivity 20 years ago, which I guess made sense at the time but you'd never see it today. I had a Rotel RSP 980 pre-amplifier, Rotel RB-985 amplifier, and Rotel's Dolby Digital decoder - connected with 2x DB-25 cables. It was fine for my LD, but when moving forward to using a HTPC around 2005, this required real time DD encoding on the PC end, something that was not always that easy to get going. Also the Dolby Digital path didn't exactly give me stellar audio quality. High compressed, and there was something about the quality of the signal that left me wanting. Even listening to stereo music meant an enforced, compressed DD path.
In particular I wanted to access DTS HD Master + TrueHD audio, so with no other multi channel inputs, the alternative would have been to buy a completely new AV setup to support DTS etc. However the thought occurred to me that the DB-25 was just an analogue connection, and what if a converter cable existed? True enough, after much searching I managed to source a DB-25 to 6 RCA connector cable, which then let me hook up an Asus Xonar HDAV1.3 Deluxe sound card.
This switch resulted in a massive improvement of sound quality, much thanks to the excellent Asus product as well, and made using a now 20 year old hifi system completely viable.