I found a different Mac ABX comparison program (MacABX, found here: http://www.cs.brown.edu/~tl/software.shtml) and used it to test. The test was done on the track "Some Other Time" from The Alan Parsons Project's album from 1976 entitled,"I Robot".
One file is lossless and the other is a 128kbps MP3 encoded with the LAME UI client Max, which is the most audiophile-accepted MP3 encoding solution on the Mac and is updated very regularly. LAME options were set to highest quality and constant bitrate (NOT VBR) of 128kbs.
The testing was done through a pair of Sennheiser HD280 Pro's, which are extremely accurate, sharp, and unbiased. They're actually one of the three top headphone choices among professional audio monitors.
This single screenshot sums up my findings. One the file to the left IS indeed lossless, but I was required to change its extension to .mp3 in order for it to be accepted in the "Choose File" dialog.
Out of 72 tests between the two, I only got it wrong twice. Seems that there indeed is a great deal of difference... In fact, it was incredibly easy because the MP3 had a distinctive distortion in the beginning that the lossless file completely lacked.