Just enough to keep MODX+ competitive in its price bracket. The bump in FM-X polyphony indicates the likely addition of a second SWP70 tone generator IC.
Yamaha have moved on from the SSP2 signal processing chip to the SSP3. The SSP3 first appeared in the Yamaha AXR4U and Steinberg UR44C audio interfaces. Yamaha move ahead like a shark and the SSP2 is deprecated for new designs. In fact, the MODX+ refresh likely includes the SSP3 as SSP2 production inventories must be low or exhausted.