From Bricasti:
"The MDx processor is our latest enhancement to the product line, this upgrade will provide for DSD256 with USB, and overall improvement over the previous revisions of the processors used in our products for digital timing. The MDx processor is used in all new production so upgrading to it will bring the product up to current production spec. In particular with the MDx there is a noted improvement when using the network interface over the previous type processors, this change is for better sync to the incoming master clock. The M3 has had the improved Master clock timing, as about a year ago at the time we introduced the M3 we made changes in all product with the previous versions of the processor ( Rev 8 ) to yield better performance. But the MDx provides even better performance than the first processor the M3 was shipped with, so there will be an overall improvement with the M3 running the MDx on all inputs. The Rev8 processor board used in the first generation M3 has excellent performance, the MDx is just a bit better in all aspects and especially when using the interface for the network.
Whether or not this is worthwhile is up to the listener. Users report that it is, but of course this becomes subjective opinion of the listener. One might describe the sound effect is of greater accuracy of transients and presentation of the imaging to be more accurate. This can be perceived as slight or profound, depends on the playback chain and source. This is like changing the sync setting in the M3 from fine ( most accurate timing ) to wide ( less accurate). With the MDX we go to super fine mode."