A good dac will make a noticeable improvement to the SQ of a computer based playback system. Getting better sound out of an ipod is a different story.
I wouldn't worry that much about a sub at this point. In any case, I don't know offhand of a dac that does 2.1 management. You could get a pre-amp to handle this. The emotiva pre-amp has pretty decent bass management.
In any case, the dac isn't about sub management but about improving the quality of the digital to analog conversion that would otherwise be handled by your computer (assuming this is your source). Are you going to keep your computer near your speakers? If you do, the HRT music streamer II is inexpensive and really good. It is powered by and uses a usb port for audio. If the computer is not going to be near the speakers, then you want something to stream your music. It's hard to beat the logitech squeezebox touch; it sounds and works great.
By the way, let me know what you think of the Swan MKIII; they look like really cool speakers.