Reddit mentions of HIFIMAN - Ef5 Tube Headphone Amplifier

There are all sorts of amps you can get. Are you interested in tube or solid-state? Are you getting the ATH-M50s for sure? If you are, you will need to get a high-wattage tube amplifier, because the ATH-M50s only produce 38 ohms, which is a low impedance. This means that plugged into a tube amplifier, they will not be very loud, especially compared to higher-impedance headphones. To compensate for this, you will need to get a higher-wattage amplifier, which translates to more expensive.

Depending on your price range, I would direct you to anything from the solid-state Behringer HA40 to the tube HiFiMan Ef5. Honestly unless you are an experienced audiophile, you will not need a tube amplifier, but they are really cool. If you want to meet somewhere in the middle there are also what's called integrated amplifiers, which combine the components of tube and solid-state amplifiers. They claim is that they produce the quality of tube with the volume of solid-state at a price between the two. That's up to the judges right now but for most an integrated amplifier is definitely ample, especially a beginner.

Personally I use the FiiO E6 as my portable headphone amplifier. It's pretty good for being so small but I do hear white noise at louder levels. FiiO is a fairly reputable amplifier company if you want to look into them. I really don't know enough about amplifiers yet (this is my weakest area) to recommend a specific product, but here are some popular integrated amplifiers. I'm still garnering the money and experience to become an amplifier connoisseur.