The Superlux HD 681 are a good sounding pair of semi-open over-ear headphones. They have a nearly flawless Bass, an excellent Mid Range and a decent Treble. They also perform quite consistently across multiple humans, but if you wear glasses there's a chance you get slightly less Bass compared to those who don't wear glasses. Additionally, they have an average Soundstage and image decently, but their mediocre distortion could make their Bass and Treble slightly harsh under heavy loads.
The HD 681 are not designed to block ambient noise. They have semi-open ear cups that create a more immersive Soundstage but also leaks and lets ambient noise seep into your audio. This means the HD 681 won't be the ideal headphones for commuting or to use at the office since you will struggle to hear your music in noisy environments and the level of leakage will be too distracting to the people around you in quieter settings.
The Superlux HD 681 are a good, budget option for critical listening. They have slightly extended bass and deliver a satisfying audio reproduction that caters well to instruments and vocals. They have a relatively flat mid-range but tend to sound slightly sharp because of the bump in the high-mid/low-treble, especially with tracks that are already a little bright. Sadly, they don't have a Soundstage as immersive as some of the other fully open-back headphones, but overall the sound quality of the HD 681 is good for both casual and critical listeners.