The MODX+ boasts 128-note FM-X polyphony, up from 64-note polyphony in MODX. It's the same spec as the Yamaha MONTAGE. You’ll be able to play more simultaneous FM-X parts without note drop.
The original MODX had a total of 1 Gb of onboard Flash Memory. MODX+ now has the same amount as MONTAGE with a total of 1.75 Gb. The increased memory allows you to play more sounds from the updated Bösendorfer 290 Imperial Sample Library.
The pitch bend and modulations wheels have been upgraded from plastic to rubber, same as the Montage. This may not seem like a big deal, but it is noticeable when playing; i.e. the rubber wheels feel more responsive and comfortable.
If you want more info, check it out here: Yamaha MODX6+