On not hearing differences... there are many factors to consider.
1. Quality of the original recording. There are many ways for this to fail, from poor-quality mics to using too much compression. Compressors have become much better in the last few years, with fewer obvious artifacts.
2. Quality of the playback chain, from transducer through amplification.
3. Quality of the MP3 conversion.
4. Sensitivity and training of your audio perception system, which includes ears, nervous system, what you've learned and what your biases are.
So, if you don't hear a difference, no problem. Don't let some golden-eared audiophile tell you what to do. Buy and use the equipment that makes you happy, not your friends or the reviewers. In truth, everything these days sounds pretty good, except speakers. Speakers are always the weakest link in the chain.
There are all kinds of different people, which is why there are all kinds of different machinery.