The threshold for "transparency" varies a great deal across users. Some people I know cannot ABX 64 kb/s Opus from the uncompressed original, yet once in a while we hear of someone being able to ABX one particular sample all the way up 192 kb/s. There's also the issue of whether you have the reference to compare. Personally, I stop being able to ABX Opus somewhere around 128 kb/s, but I won't be able to tell that a song has been compressed with Opus at 96 kb/s unless I have the original to compare it to.