The System Mix Plus (SM+) is the heart of your rack setup. They are no longer produced and are hard to find but they are worth the price. I have been using mine for years and it's a great unit. It is the heart of my rack and I'm not even using all the possibilities it gives you.
I used to run it as a rack interface but no longer do that, it worked great for that though. Then, it has a dual stereo mixer on board, allowing you to run fx in parallel, series or both with your dry tone. The mixer part is the same as the Voodoo Lab Dual Line mixer and works and sounds great. It also has a VCA on board, this is a midi-controllable volume pot. You can control the volume of the signal running thru the VCA via your midi-board. I use this as the overall master for my setup, I have it patched after the preamp and before FX so if I turn down the volume, I still have the trails from delay and reverb.
Stuff I'm not using, or not often use, are the balanced outputs L+R with speaker simulation (works good, sounds ok), the feed-thrus and the ability to plug in headphones for late night playing. My signal path runs from the pre-amp into the VCA which acts as my master volume. Then into MIX1. In Mix1 I have a FX processor, in Mix2 I have another one. The output of the mixer is then send to my power amp. Having the VCA after the preamp and before fx allows me to do volume swells and shut down the volume while still having the delay and reverb trails.
The manual is very good and explains it all. Like I said, hard to find but worth the price. If mine died, I'd be in trouble because it does so much in my rig, it is hard to replace.