Was your back cold, or were you just plain cold? The difference between those two is pretty clear when you're experiencing it. Three layers of IX with 5oz Climashield should be good to really really low temps if the IX is built differentially.
One thing you might consider for your topquilt - don't rebuild it - instead make a cover for it. This way, separated you have two quilts for summer, and together, one for winter.
There can be issues with what I'm about to recommend, but it has worked extremely well for me. I have used a 3oz Climashield top quilt along with a 2-layer IX topquilt shell (IX over the Climashield) into the 20's and was plenty warm. I tend to sleep cold. The IX traps your warmth in the climashield very effectively. But there is a caveat - IX is also somewhat of a vapor barrier, so if it's not extremely cold, or you're a heavy sweater, you can end up with a bit of a soggy climashield bag in the morning. Good news is, synthetics still work when wet, though with slightly diminished effectiveness. The worst I've ever had with this setup is a tiny bit of moisture in the foot end of my climashield quilt, which dried out in 30min hanging over my hammock in the morning. Again - this involves a bit of a vapor barrier, so that must be taken into account when working out your sleep system as it could cause significant dampness for some people.