6) iFi Zen DAC V2 ($189)
The 6 ΒΌ-inch-wide Zen DAC V2 might be the most practical and powerful way to upgrade an old stereo. It contains both a digital-to-analog converter that handles just about any type of digital audio file or stream and a powerful integral headphone amp. It accepts USB digital input from a computer or mobile device and outputs it to active speakers, a stereo system, or headphones through standard or pro-style balanced audio connections.