I have the platinum Plus which I have upgraded to the 16X filter and the 2nd Gen DACs, which I believe the III comes with standard. There is a significant step up in sound from from the standard DACs and filter. I would comment the same as Robertbrown. If you already own the Plus, it will cost roughly $3000 to bring it up to the III in performance. I have been testing the "temp" version of the latest filter software and it is quite nice, more air and extension. I will probably buy the filter for $200. From what I have been told by others, the 32X filter and Sig DACs take the DAC even further, but at a cost of over $10K I can't swing it. (even with the $1000 trade in for my 16X)
If you have really deep pockets MSB now offers a "Diamond" DAC upgrade for $14K.