You might have reading lots of reviews, still could not decide what to choose. I have tried, at least 4 highly rated amps in the price range of $100. Only the O2 amp that made me satisfied. Despite being an audio enthusiast for decades, I am new to the world of full-size headphones. I have been using Etymotic ER3SE, a reference IEM for years and happy with the sound quality so far. Recently, I decided to jump into the world of full-size headphones. Starting from the highly-rated headphones like Sennheiser HD6XX and Hifiman HE4XX. I realized these cans were much worse than the IEM either driven by a laptop or smartphone. With the Objective 2, both cans outperformed my reference IEM in nearly all departments. The sound was more controlled. For example, the rumble of the lower bass, the airy background sound, and the imaging was noticeably improved. On top of that, O2 can be used both in desktop mode and portable mode. This is useful when you want to carry your music around. The cheaper amp made the headphones louder, but I could not feel the enjoyment of listening as I heard from my reference IEM. I don't think, one should invest more money on the amp than the headphones. O2 is the optimal purchase point between budget and quality for me, strongly recommended.

I received and used Objective 2 for one week. It is a very good production. It has very low noise (can't heard), very low distortion, flat frequency response. I used it for Sennheiser HD 600. It is very good match.

Needed some extra power for my AKG 612 Pro's and this little thing delivered. Just buy this thing, you'll never regret it.