Amplifiers only "understand" analog signals, hence... if you have a digital source, there is no choice but to have a DAC, since this is the device that converts the digital signal sent by the source into an analog one that can be sent to an amplifier (DAC =
Digital to
Analog
Converter).
Every digital device, like an iPod, or a cell phone, has a DAC and Amp, and as you can imagine, most of the time, those are "generic" ones that do the job good enough (for most people).
At the same time, you can have both DAC & AMP as separate devices, which allow manufacturers to develop much more advanced (higher quality) products.
Those separate devices are available in a huge range of prices. There is a consensus regarding that at a certain price point, diminishing returns law applies.
There is no consensus at all about what's that price point, so is up to you to decide that, just using your ears
EDIT:
If you're interested in a DAC and/or an Amp, take a look at what Schiit offers...
http://schiit.com/
Those guys are known to offer kick ass products at great prices. I doubt there's another company with a better quality/price ratio.