All the Superlux cans I've tried (681EVO, 668B, 531, 631) are at the top of their tier in terms of sound vs price. If someone only has $40 or so to spend, Superlux is an excellent choice. It is very good sound - but not great. All of the Superlux cans are easily outclassed by some cans costing $200 or more. However, not all cans costing $200 or more will sound better than a Superlux, but most of the $200 will be better built.
Just my opinion. I've got a karaoke setup at a local club and I use Superlux headphones for monitoring purposes and leave them there. They sound good enough and the loss won't be so great if they walk. My more expensive cans do a better job, but I don't want to subject them to the torture of commercial use or risk them walking.