I know you probably just mistyped, but please know the difference between AB and ABX testing. The former is not useful in this context.
I think you'll find if you're ABX testing a random song for the first time, you'll probably even have trouble comparing 128 vs flac. You will be surprised how hard you need to listen, and how much experience it takes knowing where to listen. There are some rare files where it is very obvious, but on the whole lossy encodings are very good and you will find yourself confused.