Oh, and you know that what you're putting IN to the mp3 encode/downsample is going to play a big role. Extremely limited material that's right up against 0dBFS is going to give the encoder fits. It's essentially a 256 band equalizer, and when you start removing stuff and reducing the bitrate, you're not going to be able to cleanly express that stuff if you've sent it a square wave.
In Pro Tools, are you using the best quality settings, btw?
How do you back up this statement with evidence, btw? Do you make an mp3 with Pro Tools and then with Barbabatch and try a null test? I'm not sure how you'd even back this up, but if you're going to CONTINUE to bait the forum with this statement, I'd like to see you back it up. Otherwise, you're just posting noise. It's pretty well proven that if you act like a professional here, the forum members are willing to listen and give you the time of day, but c'mon. Stop, or show us what the hell you're talking about with real data.