But now I wonder, would it not be better even than a stir plate to get an aquarium pump and a sterile filter and just continously bubble air through the starter?
It sure wouldn't hurt, but be careful with the rate and way the air is bubbled. I struggled with this for years. Even though I use a 6L Erhlenmyer, I was occassionally creating 'bubble-overs' if I bubbled air through the wort. Its much worse if an air stone is used since the bubbles are much finer and frothy. I use the same low gravity wort made from DME for all my starters. I found that the tendency to bubble-over is a function of the yeast strain. Hefeweizen yeasts were particularly bad for me.
That is how I came to my current recommendation to just flood the headspace above the wort with filtered air and let the stir bar do the rest of the oxygen transfer. No bubbling this way. If a stirrer is not used, then I suppose that bubbling the filtered air through the wort is the way to go. Just be sure to use a vessel that is way bigger than the amount of wort so that bubble-overs don't make it out of the vessel. By the way, those bubbles carry a huge amount of yeast, so loosing that foam is not good for your final yeast count.