Loud records get a bad name these days and how loud you’ll want to make your own particular record is a huge creative decision. Unfortunately, many musicians don’t know how to properly make this crucial judgment call.
Standards - Every musician is going to have a different sound they’re going for. Get together a few records that you like the sound of and would like to draw comparisons to. It’s also smart to find records your producer has done so you can use them for reference. Once you have assembled these records, you have to do a proper comparison of them against your recordings.
MP3 Woes - If you’re going to be comparing MP3s to your master, make sure you have good rips of your reference material. Low bitrate MP3s or webrips do not provide accurate references. You can click to get info in iTunes to see the bit rates for these MP3s and see if they’re up to par. You also need to go into iTunes preferences and make sure SoundCheck is not on in the playback panel.
Evaluating - All of these records need to come from the same source. If you’re going to listen to your master from iTunes, you then need to listen to your references through iTunes as well. If you’re going to listen to your master from a CD, then you need to listen to your references from CDs as well. You then need to put your music on at a reasonable volume. If you have a smartphone, download a decibel meter (iOS and Android both have them) and turn up the volume of your stereo until you hit an average of 82db (for scientific reasons this is an ideal listening volume, for further reading look up “The Fletcher-Munson Curve”). If you don’t have one, turn your stereo up to a volume that you could talk loudly over the top of. Once this volume is set, don’t touch the volume knob again until the whole process is over.
Once the volume is set, put on your reference material and then put on your song (in that order). Keep doing this and take notes on what you hear. Listen back and forth for a while and make honest judgments about whether your record is as loud as the records in your reference material. In the end you should have a clear perspective on where your record stands in this process.