Adventures in Mono


I'm looking for some help understanding what the mono button on my amp is doing and why it seems to be doing something different than when I "fold down" the channels from the stereo output on my amp in Pro Tools. Below is a link to an audio clip of the intro to a 'microgroove' mono record circa 1959 being played with a Shure m44-7 on a Technics SL-1200 mk2 into a Marantz 1090 stereo amp. There are three variations of settings played consecutively and in order (digital peaks for left and right channels are in parenthesis):

1. (Stereo) Mono button off, no "fold-down" in Pro Tools i.e. left and right panning knobs on the stereo channel are panned hard left and hard left (-7.2 dB/-6.6 dB)
2. Mono button on, no fold-down in Pro Tools (-7.6/-8.1)
3. Mono button off, left and right channels "folded down" in Pro Tools, i.e. left and right panning knobs both set to center. (-4.9/-4.9)

I was surprised that pressing the mono button didn't provide the same increase in volume as folding down the channels. Does anyone have any ideas as to what's happening when this button is pressed?

I also wanted to note that, when I used the Waves Imager plug-in to separate the "mid" and "side" information, that it sounded very similar to when I simply fold down the channels. Any comments?

(By the way, I have no intention of recording my records into Pro Tools nor using the program as a mixer. The reason I did this was to see if the mono button on my amp produced the same effect as folding down the channels, which it apparently does not.)

Thanks for the help!
I don't know anything about the "folding Down" process but of the 3 versions I liked the third the most as it provided the most texture to the sound. It is some kind of DSP I take it.
I agree with Mechans, they sound progressively better from one ---> three. There is a slight sound increase as well which may make the third sample sound more clear and coherent.