Sorry, I'm not familiar with the SOL bivy, so I didn't pay attention to it. If it's not breathable, it is a vapor barrier and definitely should be inside your insulation, not outside. JustWalking is correct here; the bivy, when used outside your quilt, traps your body moisture (technically known as "insensible perspiration") in your quilt insulation! That, not just falling temperature, undoubtedly explains why you were getting cold by morning--your down insulation was getting damp and losing its effectiveness!

I've had this happen even using a breathable bivy, mostly because my tossing and turning made the waterproof underside of the bivy end up on top, achieving the same effect! Even when this didn't happen, I found that even the breathable nylon on top retarded some of the evaporation from the sleeping bag.

If you wear additional insulation inside your sleeping bag, such as a jacket, it needs to be outside the SOL bivy or other vapor barrier for the same reason. The only thing inside the bivy or other vapor barrier should be your wicking base layer, which will dry really fast--almost instantly--from your body heat after you arise and start moving around.

Most people are uncomfortable in a vapor barrier in temperatures above freezing. I'd therefore leave the SOL thing off altogether until the temps get frosty! Then use it under the quilt.

Try that simple fix before you spend more $$$ on another (and heavier) quilt or a sleeping bag!