I use a combo of oxyclean & biz. Take 1 scoop(the oxyclean scoop) of biz and oxyclean for each gallon of water. Soak for a couple hours depending on the yellowing.
I used my washer - but I also don't have a front loader, so i can use lots of water. You could also use the bath tub. I spun it out when i wanted to see if i got the yellowing out. I will say that I didn't think I had gotten the yellowing out, but when I dried the quilt, it really was gone. Go figure!