End80, thanks for the link to that file again. I dropped the file into Cool Edit wave editor, and did some listening.
Originally I had thought that the modification just brought up the bass too much, and tonal balance was off from the bass being overwhelming, but there's more to it than that.
Using Cool's Spectral View shows there are frequencies extending above 5000 hz in the file as a whole.
The modified, improved sections of the file show the frequency response stopping at about 4 khz, while the unmodified transmitter response goes up to 5 khz and a little bit more, that's an extra kilohertz of top end we're hearing!
Plus, the unmodified's top end is stronger. I used the 'FFT' function in Cool Edit to filter out everything below 3 khz in the file, and the modified unit's audio peaks hit -33 db on the meter, and the unmodified hit -22, or 11 db higher, so it's going to sound more crisp, with more and louder consonant and sibilant 'S' sounds.
As for why this happened, I don't know, I'd have to know more about the parameters of the test setup. If it was monitored on a receiver with DSP, its own internal audio filters could have even reacted differently with the different signals.
If the improved TH unit is still being offered, I'd suggest more demos with different receivers, and also different kinds of program audio.