Why is there mild distortion/fuzz on the MIDI Piano Track?

Hello, after many months mixing a mystic folk album, I am now mastering said album using Logic Pro X version 10.1.0 on a mid-2010 Macbook Pro (OS X Yosemite 10.10.5). This particular song uses mostly MIDI tracks using the Logic Pro X built-in MIDI sounds - specifically Bass Organ, Flute Organ, and Boesendorfer Grand Piano.

The intro starts with the Bass Organ, then Flute Organ, and then the Boesendorfer Grand Piano comes in playing a gentle melody. Only now during the Mastering process for this album under much closer listening with Yamaha SH5 studio monitors am I hearing a mild distortion or fuzz on the piano track. I do not hear this in my Sennheiser Headphones, but I did hear it at another studio a few days prior on Alesis M1 Active MK2 studio monitors. I hear the fuzz during playback in Logic as well as during playback from bounces, both WAV and MP3.

It seems to only occur in the left monitor. The Piano MIDI track is panned about -16 to the left, and even if I pan it far right the fuzz is still in the left monitor. Since I heard this on more than one monitoring system I believe it is occurring on the MIDI track itself.

I tried playing with EQ, multipressor, track volume, lessening the reverb, turning off the AUX sends, using different Piano Plug-ins (Yamaha Grand Piano, Steinway Grand Piano, etc.)... effecting the other tracks, like high pass filter on the Bass Organ to see if the abundance of low frequency was causing some kind of feedback or overload when in combination with the piano track... so far none of these approaches have solved this mysterious fuzz.

Furthermore, I am in the process of Mastering 11 songs for this album and none of the other songs have any of this kind of fuzz, and I can play any song from my iTunes library and I do not hear any distortion or fuzz, so I am 99% certain it is not the monitors, or anything to do with the computer. It seems to be coming somewhere from the Piano MIDI Track!!

Also, Velocity range of the MIDI notes is a gentle 48 - 61 Velocity. So it's not like the MIDI is hitting the notes too hard. I worked a lot on this song with headphones and never heard this, as it is somehow a very gentle fuzz, but it is totally noticeable upon close listening and fairly loud volume. Most of our band's audience will likely not notice it, but I cannot have this! I notice it and I want to know how to solve this.

It is a very gentle fuzz or distortion, like a mild sizzle after the Piano notes are played.

I hope that describes the situation well enough and One of you geniuses can help solve this mystery.

Namaste and Thank You!