As already mentioned, our audio devices do not support any DTS decoding, that is quite specialised and these units were not built for it.
However as quite rightly explained, our units can be used to output surround sound (2.1, 5.1 & 7.1 depending on the Scarlett/Saffire you choose), assuming that the audio program you're sending signal from utilises the ASIO driver for multiple outputs.
For example, it is quite possible to mix a song in Logic/Pro Tools/Cubase etc for surround, and, using the Mix Control software, send that out to a surround system.
Whilst the 18i20 is unable to record Loopback (audio playing on your computer) without a physical cable from output to input, our Firewire Saffire range does have the loopback feature for 'in-the-box' loopback recording, the Saffire Pro40
is the I/O equivalent of the Scarlett 18i20
Hope this helps, however it does sound like you require a slightly different type of unit to the above.
Good luck and hope you get up and running soon.
Simon // Focusrite Technical Support