Hey Arkady,
The quality of your system doesn't have to be that high; this scenario is controlled by your amp not deminishing the signal in any way and the important part - your speakers.
If you have a pair of (relatively) flat response monitors (passives go up from about £50) it doesn't matter too much on the low end response but the high end needs to extend flat to 20KHz as it's the treble that MP3 "ignores" for space, but this is where detail is.
MP3 is great for saving space or for listening to any track you want to while running/cycling/working out without it skipping. For serious audio it's got decades to go before it can compete with CD, which is why I always say "Deal in CD (buy/record/store) and convert to MP3!"
As far as monitoring goes, it is better to monitor from speakers than headphones and whatever speakers you have, good or bad you need to calibrate your ears to them - that is listen to different genres of music through them and then play your own with exactly the same volume and tone (EQ) settings and see how they compare.
Headphone monitoring requires setting up a special mix that "bleeds" just the right amount of each channels sound to the other ear.
REMEMBER: If you monitor quietly you will have the benefit of hearing what is too prominent or lacking in the mix AND save your hearing.
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