I'm comfortable recommending any of the three amps here; I suggest going with the amplifier that's best for your budget and speakers. If you want to get up and running as inexpensively as possible, the Orb Audio Mini-T will do a fine job of bringing your old speakers back to life. But for not much more money ($20 at current street prices), the Dayton Audio DTA--100a offers a notable improvement in audio quality, two inputs, and a pretty good headphone jack. The problems I encountered with stereo balance at low volumes were disappointing, but I suspect they won't be issues for most people, and I think the DTA--100a is the overall better value. If you're planning to use a computer as a source, though, you should consider the previously reviewed Topping TP30 and its built-in DAC.
For a bit more money, the Audioengine N22 offers the best construction of the three units, and by far the best sound--if you've spent $200 or more on your speakers, the N22 will help them sound their best. However, the N22 is also the same price as some entry-level stereo and home theater receivers, which will offer many more features but take up substantially more space. The N22 is also very close in price to NuForce's great $229 Dia, which adds digital inputs and a remote (but omits analog inputs). If your equipment has questionable analog outputs but coaxial- or optical-digital outputs, I recommend the Dia over the N22. With a quality analog-audio source, however, the N22 shines, and is the best desktop amplifier I've heard.
Sign up for Computerworld eNewsletters.