Here's an awesome paper all about yeast propagation, It talks about using a magnetic stirrer for a yeast starter
MB Raines, Ph.D. - Guide to Yeast Culturing for Homebrewers - Maltose Falcons Home Brewing Society (Los Angeles Homebrewing)
Nice Partial Mash Info!
You said you make 2.5 all grain beer in the same manner. How would the process differ with all-grain? I'm interested in giving that a shot.