Mackie HM-4

Don't expect the best quality from this preamp but it is definitely worth the money. The grounding in the power supply isn't the greatest so you'll get a lot of electrical noise coming through if you have this near several other power cables but easy enough to navigate around if you just move it a bit.

Other than that, this thing can push headphones stupidly loud, and the quality remains perfect until about 80% or more and then you get audible distortion, but at that level you're definitely damaging your ears anyway!

For reference, I use the amp with both Beyer DT770 Pro's (80 Ohms) and Audio Technica M50x's.

Pros:
Insane value for what it is. I upgraded my headphones to Behringer DT990 PRO 250 and I needed to power them. This gets the job done phenomenally. The knobs work smooth and there is no friction "electronic" noise when manipulating them. Plus my workspace looks a bit more complex now with another device on my desk.

"Cons" (funny):
If you keep it in the same room you're sleeping in, the green led light might be pretty bright at night. (LEDs are plaguing my setup at the moment...). This little LED seems bright enough to light up the ceiling area above it at night. I guess you can cover it with any sticker.