Mackie HM-4

I use this a lot more than I thought I would

Works great in any scenario where you need more volume in your headphones or a speaker with a headphone input.
To my ears it slightly brightens the sound, but I use it in specific scenarios so take this with a grain of salt.

I got it to raise the volume of backing tracks when I'm drumming on my electric drumset, did the job.

I found myself using it to listen to music on my tablet with the studio headphones, WOW
It's now my favorite way of listening to music, when I want to hear the details.

Also use it to play podcasts, videos and music through my bass amp, it's just convenient.

Strong metal casing, knobs feel kinda cheap but work great.
Looks cool as well, I have to say

Negative aspects:
It's small and a bit heavy, with large jacks and cables plugged in it has a tendency to not be stable and fall.
That's the one note I have to give, just be careful so it doesn't fall and you are good.

Great product, my favorite studio tool

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.