I've been using my iPod Touch and a Pure i-20 dock with a digital output. My digital cable is a 1.5m Atlas Opus but you don't have to spend that much. (For example, you could use the cheaper Atlas Compass or one of Joe Mazzaglia's excellent Auricle digital cables.) Just make sure the cable is 1.5 meters long, and run it from the Pure's coax out to the Cambridge coax in.
Verify at pure.com/us/ that your iPod is supported. If it is, this is how you will get the best possible sound (using 16/44 music files on your iPod) for the least money.
Using your computer as a source requires taking a USB output to the 740C's S/PDIF coax input, in most cases. If your computer has an S/PDIF output you're in luck, and some folks' computers also have an optical S/PDIF out, which will also work. If all you have is USB, though, you will need something to convert your USB signal to S/PDIF coax. A popular one at a medium price is the M2Tech HiFace, but you can pay less and also much more.