Sajjad bhai I will explain with the help of my experience since going into technicalities will be a length discussion.
The difference would not be fair since I have been using different equipment on each head unit. SM + Morel Elate (3way) + McIntosh vs. 80PRS + Morel Elate LE (2way) + PDX F6. By experience I can say the earlier setup trumped all over the current setup I still own. The sound signature was warm and the sound stage was huge and pretty dramatic. It has great reproduction in the lower frequencies while it just could not miss out any tiny details in the upper portion of the higher frequencies as well. With the presence of McIntosh it was just another world. The instrument separation was much more vivid.
SQ wise I rate the Sound Monitor with the likes of Clarion HD-X2, McIntosh MX5000, Denon DCT-100, F1 Status, Panny Bottlehead, etc and way above the current HU's by Pioneer (flagships included).
Technology and features wise I rate the 80PRS or for sure the P99rs way above the SM. (apart from parametric EQ and build in pink noise generator).
So yes SM was something unique and if someone has the opportunity to grab it, one shouldn't miss it. However I moved on since I got a great deal for it and wanted good audio collection in hand at all times (USB + IPOD). SM with a DSP would be lethal